Spam on the rise and I'm not sure why

[Chimpie]

I'm guessing it's a particular person, but over the past couple of weeks spam has been increasing in my forum. While the spam has been somewhat similiar in nature, the IPs and email addresses have been completely different. I'm looking for some trouble shooting tips.

Since it's creation I've had image verification on and installed the NoSpam! mod and change the questions about every other week if not more frequently.

What else should I be looking at or should I install?

Chimpie