Hi haddock,
Thanks for the link! Unfortunately it shows that nearly everything that we could do against forum spammers has so far been done. But maybe you're right with the idea of an admin manually approving each user registration. If we do some outsourcing this may lead to an interesting kind of business model: the same guy who is paied to visually solve our captcha can later reject his own registration attempt...

Cheers,
Helge