Thanks for the help, George.
Chee, these users are coming from similar IP addresses, but they are different enough that I can't block out a huge chunk. If I did that, I might prevent an actual user from getting to the forums.
I've enabled Google's reCAPTCHA on the registration page. It's supposed to be tough stuff. I was going to wait until v3 to add that, but decided to go with it now anyways.
Thanks for helping us out with this issue everyone. Keep posting spammers here. This should be over soon.