For another project, I run a simple bulletin board using phpBB. Over the past several months it’s basically been unusable due to spam. I think the bots are able to defeat the standard captcha that phpBB provides. I tried a number of solutions (increased requirements for posting, hidden fields, reducing the reward for spamming, etc.), but nothing seemed to work very well at reducing the spam volume.
Thankfully, I think I’ve got it licked now, at least for the time being. Anti-Spam ACP seems to be doing the trick. It’s showing in its logs around 150 spammer registrations per day that are being denied. It’s nice not having to manually delete all that junk!