I'll email bustin about it tonight. I admit I'm not that competent when it comes to forum stuff, so he's the man for the technical stuff.
What I have been doing recently, though, is manually looking at new members before even asking for the required activation. As I look at the upcoming members list, I use my own judgment to remove those with suspicious-looking email addresses and I also sort them by IP to remove them if there are multiple ones from that same IP address. And then when there is trouble, instead of just banning a user by name, I now ban the IPs they register and post with, and all the other possible ban trigger options relating to that username and IP. I really should have done that before. It's not going to stop the really persistent ones, but it might help a little until bustin implements upgrades to the forum software.