Is it possible you could make a post with guidelines on how the forum should be moderated ?.
I noticed on my other forum, that if i banned spammers for a period of time, they come back a month, 3 months later, 6 months, later. I assume that a permanent ban is best. We only had one confirmed false banning. Which i guess is pretty good ratios to actual spammers. Even if another one was to exist in the future, they can view the forum, and view that the only alternative is to join the IRC channel to get a fix.