BuddyPress.org

I would try to prevent member registration through .htaccess:

http://wpmututorials.com/how-to/spam-blogs-and-buddypress/

If not .htaccess, then try WP Hashcash, but I prefer the simplistic method of blocking through .htaccess. This will block most spam signups, but of course you can’t block everything!

Thanks to Darcy Norman for the original tip!