There's a great hack for this for phpBB.
http://www.phpbb.com/phpBB/viewtopic.php?t=248252
You can download the zip file there.
It's actually 2 hacks, one references the Distributed Server Boycott List (DSBL) and compares IP's, while the other does an actual scan for open proxies.
The author has given permission for it to be ported to vBulletin as long as you mention him in the credits.