Spam filter for registration with possibility to detect real users

Hi

We started to have trouble with a lot of spam bots spamming down our forum. The solution was to try a plugin which stops them. I’ve tried several plugins now and they do 98% good job of stopping them and that’s good enough. The problem is to seperate the spam users from normal users. How have you solved this problem? Is there a plugin with a question they have to answer maybe or any other way of detecting real human users?

The page I need help with: [log in to see the link]