I think a very simple thing to try, is disable new memberships. I suspect doing that would reduce the spam to near zero, wouldn't it?
That is what I've done on a forum I host, it eliminated spam completely.
I'd be happy to take over the hosting of
http://empegbbs.com/ , I already host the other forum I mentioned and there is plenty of spare capacity on the Linode instance I use for that. If I did host it though, I'd almost certain close it to new members (or at least automatically signed up new members).