Excuse my ignorance here. I don't hang out here too much anymore, but am also getting the garbage.
Are these real people or bots getting past the captcha? If they're bots you could replace the captcha with something like this (Sample Below). It drive's 'em nuts. I've done this for a few other forums. The other sites that I do work for were getting close to 100 bot accounts a day. Once I changed to this style of authentication the number dropped to 0. It does require directly editing the forum's code, but worth the effort. If they're real people, the only other thing that can really have an affect is to require an admin approval for new accounts. What a P.I.T.A. that is.
Just my 2¢