Ok thanks.
Its interesting to know how other fight against spam. And its relieved to hear that bigger forums also has problems with spammers.
I installed a addon : VSa - Advanced Registration (im not allowed to post urls yet for the one who search for that: you can find it on the official vbulletin-addon page)
This captcha addon shows different questions you can generate by yourself.
I think for the first step this is enough for a little forum like my one.
For the other way to handle it with different databases and so on. Thats very interesting for me. But for the first, i think, i dont need so complex thinks like that. Maybe a little bit later i will do somethink like that
Thanks for your anwser.