I just checked out your registration process. It is very weak. Your word recognition validation process in your registration is not enough anymore, instead, you should use simple questions that bots cannot answer.

The best way to prevent spam is to not allow spam bots to even register. Spam programmers are going to take the time to get around Google's gmail validation, but they are not going to take the time to crack individual sites like this. It is very easy to insert code into the registration process to prevent spam bots.