When someone registers, why not have a confirmation email sent to their listed email address, which they have to click a link on to activate their accounts? Thats how most places do it.
I posted a reply in a topic in the bug section just now, and noticed the first topic there was spam advertising porn sites. Guess no moderators are online right now.
After having this mentally unstable brat voted out of my forum, I started making everyone get approval before being able to post, since he kept registering using free email host, and there far too many of them to ban. So I just take the IP address of anyone awaiting validation, and google it between quotation marks. If hundreds or thousands of things pop up, usually with the word "proxy" then I know its a proxy used by spammers, stalkers, pedifiles, and jerks in general to avoid detection. They have a lot of websites out there that allow you to do things through their IP address, so no one can trace you. I assume only degenerates use such things.
Just make sure you don't just google the numbers, since nothing shows up. You have to put them between "quotation marks". Eventually, all those trying to harass you will get bored, them stopping by for a random attempt to get in from time to time, but things will quiet down after a few days.