I'm going to give a quite simple answer: we don't do that.

First off, it wouldn't help much. There's not anything stopping him from registering another name, and starting all over again. We could do a little to make it harder, but nothing that couldn't be gotten around with siginificant ease, or cause significant bad side-effects.

Second off, it's not a good thing to do. Most trolls (and make no mistake, he is one) eventually change their ways, I think. We should be encouraging that to happen, not giving up on it ever happening.

This isn't to say that we shouldn't do things to discourage him from acting like an ass. We should. But the problem is that the way to do that, from a programatic viewpoint, is to remove the negitive feedback that's being taken by the troll as any-feedback-is-good-feedback, we also remove the negitive feedback from the times when he's actually going to take that as negitive feedback. Everybody has some of both in them, and changes between them in unpridictable ways.