A skewer, in fact!

The way to approach someone who starts out with ad hominem as the substance of his (or her) argument is to point out the various instances where it occurs and let it be known the argument is "full of sound and fury, signifying nothing."

If there's any substantive position, though, be sure to find it and address it. Otherwise, you still lose the point.

Let it be known that "Bush told a lie", with substantiation, is not ad hominem, but "Bush is a liar", with or without substantiation, is, because it attacks his character and not his action. He can't have lied about everything.

Regards--
Mary Anne