Cases like that are tough. You could turn on admin approval for new registrations.
Banning a block of IP's isn't a terrible thing. Don't be afraid of it.
When we've had idjits like that in the past, I use the ARIN database to find their domains from several posts. I always hope I find them posting from a work computer. Then a quick phone call to the employer usually takes care of the problem.
Lastly, if you could get members to just ignore this person, s/he will eventually tire of it and go away. They always do, some take a little longer than others.
