Here's how to do it for Linux:

Include the following inside /etc/hosts :

# Anti Ad IP numbers
127.0.0.1 Ogilvy.ngadcenter.net
127.0.0.1 ResponseMedia-ad.flycast.com
127.0.0.1 Suissa-ad.flycast.com
127.0.0.1 a32.g.a.yimg.com
127.0.0.1 ad-adex3.flycast.com
127.0.0.1 ad.adsmart.net
127.0.0.1 ad.ca.doubleclick.net
127.0.0.1 ad.de.doubleclick.net
127.0.0.1 ad.doubleclick.net
127.0.0.1 ad2.doubleclick.net
127.0.0.1 ad.fr.doubleclick.net
127.0.0.1 ad.jp.doubleclick.net
127.0.0.1 ad.linkexchange.com
127.0.0.1 ad.linksynergy.com
127.0.0.1 ad.nl.doubleclick.net
127.0.0.1 ad.no.doubleclick.net
127.0.0.1 ad.preferences.com
127.0.0.1 ad.sma.punto.net
127.0.0.1 ad.uk.doubleclick.net
127.0.0.1 ad.webprovider.com
127.0.0.1 adcontroller.unicast.com
127.0.0.1 adcreatives.imaginemedia.com
127.0.0.1 adex3.flycast.com
127.0.0.1 adforce.ads.imgis.com
127.0.0.1 adforce.imgis.com
127.0.0.1 adfu.blockstackers.com
127.0.0.1 adimages.earthweb.com
127.0.0.1 adimg.egroups.com
127.0.0.1 admedia.xoom.com
127.0.0.1 adpick.switchboard.com
127.0.0.1 adremote.pathfinder.com
127.0.0.1 ads.admaximize.com
127.0.0.1 ads.bfast.com
127.0.0.1 ads.enliven.com
127.0.0.1 ads.fairfax.com.au
127.0.0.1 ads.fool.com
127.0.0.1 ads.hollywood.com
127.0.0.1 ads.i33.com
127.0.0.1 ads.infi.net
127.0.0.1 ads.link4ads.com
127.0.0.1 ads.lycos.com
127.0.0.1 ads.madison.com
127.0.0.1 ads.mediaodyssey.com
127.0.0.1 ads.msn.com
127.0.0.1 ads.ninemsn.com.au
127.0.0.1 ads.seattletimes.com
127.0.0.1 ads.smartclicks.com
127.0.0.1 ads.sptimes.com
127.0.0.1 ads.tripod.com
127.0.0.1 ads.web.aol.com
127.0.0.1 ads.x10.com
127.0.0.1 ads.xtra.co.nz
127.0.0.1 ads.zdnet.com
127.0.0.1 ads01.focalink.com
127.0.0.1 ads02.focalink.com
127.0.0.1 ads03.focalink.com
127.0.0.1 ads04.focalink.com
127.0.0.1 ads05.focalink.com
127.0.0.1 ads06.focalink.com
127.0.0.1 ads08.focalink.com
127.0.0.1 ads09.focalink.com
127.0.0.1 ads1.activeagent.at
127.0.0.1 ads10.focalink.com
127.0.0.1 ads11.focalink.com
127.0.0.1 ads12.focalink.com
127.0.0.1 ads14.focalink.com
127.0.0.1 ads16.focalink.com
127.0.0.1 ads17.focalink.com
127.0.0.1 ads18.focalink.com
127.0.0.1 ads19.focalink.com
127.0.0.1 ads2.zdnet.com
127.0.0.1 ads20.focalink.com
127.0.0.1 ads21.focalink.com
127.0.0.1 ads22.focalink.com
127.0.0.1 ads23.focalink.com
127.0.0.1 ads24.focalink.com
127.0.0.1 ads25.focalink.com
127.0.0.1 ads3.zdnet.com
127.0.0.1 ads3.zdnet.com
127.0.0.1 adserv.iafrica.com
127.0.0.1 adserv.quality-channel.de
127.0.0.1 adserver.dbusiness.com
127.0.0.1 adserver.garden.com
127.0.0.1 adserver.janes.com
127.0.0.1 adserver.merc.com
127.0.0.1 adserver.monster.com
127.0.0.1 adserver.track-star.com
127.0.0.1 adserver1.ogilvy-interactive.de
127.0.0.1 adtegrity.spinbox.net
127.0.0.1 antfarm-ad.flycast.com
127.0.0.1 au.ads.link4ads.com
127.0.0.1 banner.media-system.de
127.0.0.1 banner.orb.net
127.0.0.1 banner.relcom.ru
127.0.0.1 banners.easydns.com
127.0.0.1 banners.looksmart.com
127.0.0.1 banners.wunderground.com
127.0.0.1 barnesandnoble.bfast.com
127.0.0.1 bizad.nikkeibp.co.jp
127.0.0.1 bn.bfast.com
127.0.0.1 c3.xxxcounter.com
127.0.0.1 califia.imaginemedia.com
127.0.0.1 click.avenuea.com
127.0.0.1 click.go2net.com
127.0.0.1 click.linksynergy.com
127.0.0.1 cookies.cmpnet.com
127.0.0.1 cornflakes.pathfinder.com
127.0.0.1 counter.hitbox.com
127.0.0.1 erie.smartage.com
127.0.0.1 etad.telegraph.co.uk
127.0.0.1 fp.valueclick.com
127.0.0.1 gm.preferences.com
127.0.0.1 gp.dejanews.com
127.0.0.1 hg1.hitbox.com
127.0.0.1 image.click2net.com
127.0.0.1 image.eimg.com
127.0.0.1 images2.nytimes.com
127.0.0.1 jobkeys.ngadcenter.net
127.0.0.1 kansas.valueclick.com
127.0.0.1 leader.linkexchange.com
127.0.0.1 liquidad.narrowcastmedia.com
127.0.0.1 ln.doubleclick.net
127.0.0.1 m.doubleclick.net
127.0.0.1 macaddictads.snv.futurenet.com
127.0.0.1 maximumpcads.imaginemedia.com
127.0.0.1 media.preferences.com
127.0.0.1 mercury.rmuk.co.uk
127.0.0.1 mojofarm.sjc.mediaplex.com
127.0.0.1 newads.cmpnet.com
127.0.0.1 ng3.ads.warnerbros.com
127.0.0.1 ngads.smartage.com
127.0.0.1 nsads.hotwired.com
127.0.0.1 ntbanner.digitalriver.com
127.0.0.1 ph-ad05.focalink.com
127.0.0.1 ph-ad07.focalink.com
127.0.0.1 ph-ad16.focalink.com
127.0.0.1 ph-ad17.focalink.com
127.0.0.1 ph-ad18.focalink.com
127.0.0.1 rd.yahoo.com
127.0.0.1 redherring.ngadcenter.net
127.0.0.1 redirect.click2net.com
127.0.0.1 regio.adlink.de
127.0.0.1 retaildirect.realmedia.com
127.0.0.1 s2.focalink.com
127.0.0.1 spin.spinbox.net
127.0.0.1 static.admaximize.com
127.0.0.1 stats.superstats.com
127.0.0.1 sview.avenuea.com
127.0.0.1 tracker.clicktrade.com
127.0.0.1 tsms-ad.tsms.com
127.0.0.1 v0.extreme-dm.com
127.0.0.1 v1.extreme-dm.com
127.0.0.1 van.ads.link4ads.com
127.0.0.1 view.avenuea.com
127.0.0.1 w113.hitbox.com
127.0.0.1 w25.hitbox.com
127.0.0.1 web2.deja.com
127.0.0.1 webads.bizservers.com
127.0.0.1 www.PostMasterBannerNet.com
127.0.0.1 www.ad-up.com
127.0.0.1 www.admex.com
127.0.0.1 www.alladvantage.com
127.0.0.1 www.burstnet.com
127.0.0.1 www.commission-junction.com
127.0.0.1 www.freestats.com
127.0.0.1 www.imaginemedia.com
127.0.0.1 www.netdirect.nl
127.0.0.1 www.oneandonlynetwork.com
127.0.0.1 www.targetshop.com
127.0.0.1 www.teknosurf2.com
127.0.0.1 www.teknosurf3.com
127.0.0.1 www.valueclick.com
127.0.0.1 www2.burstnet.com
127.0.0.1 www4.trix.net
127.0.0.1 www80.valueclick.com
127.0.0.1 z.extreme-dm.com
127.0.0.1 z0.extreme-dm.com
127.0.0.1 z1.extreme-dm.com

Next backup the file ~/.netscape/preferences.js and then edit ~/.netscape/preferences.js to include the following single line (you might have to delete the original no_proxies_on line):

user_pref("network.proxy.no_proxies_on", "Ogilvy.ngadcenter.net ResponseMedia-ad.flycast.com Suissa-ad.flycast.com a32.g.a.yimg.com ad-adex3.flycast.com ad.adsmart.net ad.ca.doubleclick.net ad.de.doubleclick.net ad.doubleclick.net ad.fr.doubleclick.net ad.jp.doubleclick.net ad.linkexchange.com ad.linksynergy.com ad.nl.doubleclick.net ad.no.doubleclick.net ad.preferences.com ad.sma.punto.net ad.uk.doubleclick.net ad.webprovider.com adcontroller.unicast.com adcreatives.imaginemedia.com adex3.flycast.com adforce.ads.imgis.com adforce.imgis.com adfu.blockstackers.com adimages.earthweb.com adimg.egroups.com admedia.xoom.com adpick.switchboard.com adremote.pathfinder.com ads.admaximize.com ads.bfast.com ads.enliven.com ads.fairfax.com.au ads.fool.com ads.hollywood.com ads.i33.com ads.infi.net ads.link4ads.com ads.lycos.com ads.madison.com ads.mediaodyssey.com ads.msn.com ads.ninemsn.com.au ads.seattletimes.com ads.smartclicks.com ads.sptimes.com ads.tripod.com ads.web.aol.com ads.x10.com ads.xtra.co.nz ads.zdnet.com ads01.focalink.com ads02.focalink.com ads03.focalink.com ads04.focalink.com ads05.focalink.com ads06.focalink.com ads08.focalink.com ads09.focalink.com ads1.activeagent.at ads10.focalink.com ads11.focalink.com ads12.focalink.com ads14.focalink.com ads16.focalink.com ads17.focalink.com ads18.focalink.com ads19.focalink.com ads2.zdnet.com ads20.focalink.com ads21.focalink.com ads22.focalink.com ads23.focalink.com ads24.focalink.com ads25.focalink.com ads3.zdnet.com ads3.zdnet.com adserv.iafrica.com adserv.quality-channel.de adserver.dbusiness.com adserver.garden.com adserver.janes.com adserver.merc.com adserver.monster.com adserver.track-star.com adserver1.ogilvy-interactive.de adtegrity.spinbox.net antfarm-ad.flycast.com au.ads.link4ads.com banner.media-system.de banner.orb.net banner.relcom.ru banners.easydns.com banners.looksmart.com banners.wunderground.com barnesandnoble.bfast.com bizad.nikkeibp.co.jp bn.bfast.com c3.xxxcounter.com califia.imaginemedia.com click.avenuea.com click.go2net.com click.linksynergy.com cookies.cmpnet.com cornflakes.pathfinder.com counter.hitbox.com erie.smartage.com etad.telegraph.co.uk fp.valueclick.com gm.preferences.com gp.dejanews.com hg1.hitbox.com image.click2net.com image.eimg.com images2.nytimes.com jobkeys.ngadcenter.net kansas.valueclick.com leader.linkexchange.com liquidad.narrowcastmedia.com ln.doubleclick.net m.doubleclick.net macaddictads.snv.futurenet.com maximumpcads.imaginemedia.com media.preferences.com mercury.rmuk.co.uk mojofarm.sjc.mediaplex.com newads.cmpnet.com ng3.ads.warnerbros.com ngads.smartage.com nsads.hotwired.com ntbanner.digitalriver.com ph-ad05.focalink.com ph-ad07.focalink.com ph-ad16.focalink.com ph-ad17.focalink.com ph-ad18.focalink.com rd.yahoo.com realads.realmedia.com redherring.ngadcenter.net redirect.click2net.com regio.adlink.de retaildirect.realmedia.com s2.focalink.com spin.spinbox.net static.admaximize.com stats.superstats.com sview.avenuea.com tracker.clicktrade.com tsms-ad.tsms.com v0.extreme-dm.com v1.extreme-dm.com van.ads.link4ads.com view.avenuea.com w113.hitbox.com w25.hitbox.com web2.deja.com webads.bizservers.com www.PostMasterBannerNet.com www.ad-up.com www.admex.com www.alladvantage.com www.burstnet.com www.commission-junction.com www.freestats.com www.imaginemedia.com www.netdirect.nl www.oneandonlynetwork.com www.targetshop.com www.teknosurf2.com www.teknosurf3.com www.valueclick.com www2.burstnet.com www4.trix.net www80.valueclick.com z.extreme-dm.com z0.extreme-dm.com z1.extreme-dm.com");

This procedure will stop most but not all ads. For example, go to the Dilbertzone and you'll find that most of the ads are gone. However there will still be ads appearing at MSNBC.

Gary