thanks for your suggestion, we are using multiple anti-spam measures, i thought they will be enough to keep spammers away. more or less it was a success, since there were not too many of them around.
that means one spammer/month roughly.
still, i did some steps forward in the direction to this brave new spamless world we all crave.
updated forums to newest version (we were 2 versions behind), and introduced the fancy keycaptcha service, i'm very interested if it will keep these bots away.
(btw how bots can register is beyond me. recaptcha, email verification, spam database filtering... they surely are exploiting some security holes in the forum engine)
hope this works out eventually. have a wonderful week, y'all!