I'm going to bump this thread a bit, since the problem seems to be returning a bit.
A bit of Googling yielded this mod for the version of vBulletin that CD is running that can block links in signatures of users with under X posts:
http://www.vbulletin.org/forum/showthread.php?t=123002
I don't know if it's something that's worth pursuing, but fixes (other than playing whack-a-mole) are out there.