OK, that's about 800 names deleted. While checking these, I found most of them came from the same few known spam IPs (checked against
http://www.stopforumspam.com/spamdomainsandips and other sources). I've deleted all of the old inactive accounts, and banned known spam IPs when I found one being used. This has decreased the number of fake accounts being set up. I've started checking every new account's IP for spam activity on other boards. I'll delete and ban these when I find any. I'm very suspicious about some of the newer accounts here that haven't posted, but if I haven't found any evidence they are up to no good, I'll give them a chance. If they remain inactive too long they will eventually be deleted too.
Of course, anyone who posts spam will have their account deleted and banned as soon as any mod sees it.