Was I the only person who read this and thought it could be "Dear mean people" rather than "Too many guests"?
Regardless, the best solution I've found for my low traffic site is to use a hard captcha for each post when the user isn't registered - and I also get an email notification when this happens (not often, bear in mind this is a commenting-on-posts system rather than a forum, so may not scale up.)
If you really want to allow posts from unregistered users and your sight is higher traffic, I'd throw a hard captcha on there, put an automatic spam filter on there, perhaps pass through a blacklist check (Blacklist Check
) for good measure, disallow all posts containing images or URLs, and if it passes all of those criteria have it send you a notification with the message with a quick link in the email you can use to ban or allow on sight. If you ban then add that IP to a blacklist.
Given all of the above, I'd say it's not really worth it for the small, un-loyal user base you'll gain from allowing anonymous posting - it's a lot of work for potentially very small gains.