I've also had a problem where editing the Guest permissions so they can't post comments doesn't work. I'm doing this in Administration -> User Groups -> Edit - Guest -> Post replies: No. Is this correct?
I have guest posting turned on for my forum. Three days ago I got hit by a spambot that posts 8 links per post. I added a quick-fix for that particular bot with a counter, and the next day, my counter showed that 1,503 guest posts had been blocked! I've been refining my blocker, and now prevent any hard links (that don't use the BBcode "url" tag) from working with guest accounts. I'll see how that works. If anyone is interested, I could post the code I have, as well as any other things I find out about the bots attacking my site.
Of course, this is only for bots, not people. 