Simple. Tell him he's right.
Tell him that he's right and that you are "getting angry" and obviously can't have a decent conversation about
It's amazing how often calling their bluff works. The man I used this tactic against Sunday night is still freaking out that I won't discuss politics with him until the election is over.
Why did this work, you ask? Also simple. He was cheating, of course. This man regularly comes to me and starts arguments, which I usually indulge him in for various reasons (mostly because I enjoy a good philosophical scrap). But when the topic gets a little too close to something he can't argue his way out of, he either switches topics mid-stream or else accuses me of being too aggressive. In fact, he constantly accuses me of being angry at him for one thing or another, and quite frankly, I'm sick to death of it. So I've decided not to be his enabler any more.
Why bother posting this? Well, it seems to me that there's a deeper lesson here. By arguing with him, I give tacit acceptance of his positions. If I dignify his ramble of the moment with a response, it makes him feel as if he's said something that's probably clever and definitely important. He's usually wrong on both counts.
I will give him his props, though. He did score a good point about Sarah Palin on me the other day. Well, he said something I hadn't thought about, at least. And I gave him his due on that point. But I've decided to treat him just like any other wackaloon fundagelical that wants to debate a science-minded person, be it on a stage in front of hundreds or in a hallway at two AM because he's bored. I'm going to ignore him.