The MS and Outlook plug ins are basically filters for their mail client. They usually work on keywords and phrases along with user participation. It is different than blocking spammers on a web site.

As Robert said in the thread below, the captcha method is currently pretty effective in blocking automated spamming. It does not prevent the hiring of cheap labor to bypass the simple captcha.

More and more complex captchas can be used to discourage the human spammers.

I am not aware of any programs that can block a determined spammer.

All locks are made to admit certain people and any website that allows online access by a broad base of individuals is passing out keys to strangers.

There is no way to stop it. You can only slow it down and hope they don't have sufficient reason to ruin your website and will look for easier pickings.

The human spammers that bypass captchas are paid for piecework. Their employer does not tell them to spam the Zone. They just pay them $ X for X spams. Our simple captcha means that as long as there are sites without captchas they are easier money. If we chose to use a more complex captcha, we might stop them entirely but we would also be effectively blocking potential new members. I have personally given up registering at a few sites after resetting the captcha 2-3 times without getting it right.

You will notice that those complex captchas are on high traffic sites that the spammers WILL be directed to and where they probably command a higher per piece rate.

Anyhow, the better the methods to block them, the better they will become at defeating those methods.

No doubt there are experts that know a lot more than me.

And...

Caveat: Some of what I have written above is based on educated guessing