Hi.
I have a similar problem on both nervejam.com, and the crumar yahoo group.
The forum at NerveJam.Com uses phpbb3, which allows the use of captcha (the numbers and letters verification thing). Before that, I used to approve or reject members manually: about 100 a day..
Even now, some determined spammers get through, but since there are at least 5 moderators, their messages don't last long. I have also used the "word censor" system to automatically replace some phrases: eg: "Britney Spears" becomes "John Prescott". Think: John Prescott naked.... shudder.. You don't want to know what Viagra gets replaced by! There's a warning that all posters are open to ridicule and abuse by moderators and the system itself..
PHPBB also allows you to ban mail domains, and words within email addresses. I banned Rumania..
On the crumar group, I allow anyone to join, but approve all posts manually. That's fine, as there are a small number of posts. That group used to get 100+ spam messages a day, until I took it over (the "owner" had disappeared).
Only had three such attempts in the last 6 months.
Anyway, I'm not sure what this forum runs on, but it'll have something similar to phpbb3.