I've now changed the CAPTCHA for registering, hopefully no bots will get in anymore.
This is not easy. Of course we would like that no spambots get on the forum, however we would not like to discourage people from joining the forum and posting their questions.
A surefire way would be to require all new accounts to be hand checked by an admin before users can post. However we are just not available 24/7, so this would mean a lot of delay, hours minimum, but it might even be a day. That's just not something which would work...
The new CAPTCHA is reCAPTCHA by Google, it should be quite strong.