Here is a screenshot of some spam I am getting on my website. What do you notice? I noticed that the email addresses are random letters and numbers and the names are the same category. This could be a very crude and a first way to filter out Spam. However, I believe that this forum uses Askimet. As soon as I installed and activated that, I didn't have to worry about spam, and in face, I just disabled the requirement for comment moderation because I haven't had a single spam get through.
In Short, If an email is just a weird string of character, have the user trying to sign up have to go through more recognition, same with the name of the user.
Also, I can imagine that a spammer might, himself creating an account/set of accounts for the spambots to use to get into the system. Then, the user can be disabled by a human who can detect that something is spam.
Also, Why do you have those two ads at the top. They never change, so they become kind of annoying, and I am pretty sure the "donate" button has been clicked many times because of how useful this forum is
