Jason...perhaps it's time to move to a newer version of vBulletin? I've been doing a bit of research on version 3.0.7 and it seems to have several identified security vulnerabilities which allow SQL injection and code execution. This may be being exploited by these spammers to create new user accounts and subsequent threads:
http://www.cvedetails.com/vulnerabil...tin-3.0.7.html
Also, in talking with users who are also on other forums, I've hearing that they traffic Elcova less frequently because it's not mobile device-friendly. The newer versions vBulletin would bring that functionality to the forum.