rogerd - 7:02 pm on Jul 8, 2010 (gmt 0)
vBulletin has the global ignore feature (aka Tachy goes to Coventry") built-in. It works well if the user stays logged in under that identity.
I've seen problem members who were so self-absorbed they participated in discussions with long-winded rants, and never realized that nobody responded to them directly.
Another nifty plugin lets admins set users to "miserable" and gives those users ultra-slow page loads, database error messages, timeouts, etc. This works at the user or IP level.
Ultimately, it's next to impossible to keep a problem user out if they have even a modicum of tech savvy. In my experience, though, they tire of the game when they have to keep registering new email addresses and their posts vanish as soon as they are spotted. The best solution (assuming you have tried being nice, coaching the user, etc. and that failed) is swift and consistent banning and post removal.