How to protect registration from bot's

Hi guys....

It seems that some bot is registrering users at my site. My site is a member site for members and i am not interested in having other users than those..
Is thare a smart little thing that can make users to give a proof of humanity... Like an equation or something like that,- for now i only want it in the registration form,- maybe later also when publishing various kinds of content..
Another question is,- would it be possible to make one of the required filds in the registration form depending on a logig... E.G. must be a number between 30000 and 999000
:slight_smile: Per