It's done. The thought occurs to me though, that when someone registers anew, we have no way of telling whether they'll be one of these trolls until they actually post something. So it seems to me that even though I suggested this course of action, we're not really solving the problem. The only way to avoid it is to shut down registration altogether, and while I see no immediate consequence to that, long-term it'll be a forum-killer.
If it's the same person overcoming the IP address ban, that means they're using some means to navigate to the site with a different IP address each time they register. Not sure how it all works, but something like a proxy to mask the IP from which they're actually located. Have no idea what to do about it.