It's a tempting idea, but there are so many reasons why it wouldn't work--and it's not really necessary anyway. 

If you look in Meta, you can easily tell who the trolls are and that's where most of them hang out I think.  Giving them a visible label is sort of superfluous IMO, but restricting them from starting new topics isn't a half-bad idea.

The main problem that I can see is that such a system would make the forum more of a political battlefield than it already is.  If a member and all of his alt accounts can tag a member as a troll, then you know damn well it's going to be abused.  Some members might end up with the tag just out of spite and not because they're truly trolls.

Anyway, I'd love to hear what others have to say.  I don't think Theymos would go for it, however.