Well, the rise in spam has gotten me to look more in-depth at it, and I went ahead and added a spammer database check on IP and username at registration. Now I need to go through and clean out as many spam accounts as I can find. I'm pretty sure the number is in the 1000s, but I'm not sure at this point how to be certain. I might simply have to purge any non-posters that haven't logged into the forums in, say, the past 6 months.
I hate to potentially ban legitimate users, but deleting spam and banning users is becoming a full-time job.