Having administrated an even smaller forum with a large body of spambots, I can say that disabling the guest account did wonders for the forum. (So does keeping a backup of the entire directory.) It made it much more enjoyable. The option we tried before sorta worked, but became tedious; the forum was small enough, and its members old enough, that we elevated all currently conscious posters to moderator status.
It was a little like having a squad who has been together for years and giving them all command status. No harm done, nor any particular change, just... well, we all had a lot of work to do, let's put it that way.
If you want the guest
posting still active without requiring people to type in a strange, stylized images, it's my observation that you'll need at least one more person with sufficient moderation rights to delete guest posts. We didn't have that where I was, so we just edited them to say something utterly obscene, or some such...
So yeah, I recommend that you either turn off guest posting, or get one more moderator to help keep the place clean. Spam bots accrue, and they don't go away.
A side benefit to turning off guest posting is useful for those whose computer ditches cookies: it'll tell them when they're out of their account, instead of posting it anonymously.
