By the way, I don't remember because it's been a while since I registered, but are there any anti-spammer measures in place there? You know, visual recognition codes, questions like, "are you a spambot", that sort of thing. There's also the last resort option of requiring all forum registrations to be personally approved, but that might be even more work than just permabanning spammers when they appear.
Also, I saw someone complain that they had to register to read the forums, is that true? If so I'd advise switching off that feature, nothing seems to annoy people more than not being able to look at threads in a forum. Conversely, seeing what's there encourages more people to register.