People continue to come to this forum and ask how to stop the spam messages they're receiving from bots posting as guest users. Here are your options, in one post, so you don't need to read through nine pages and find them.
Disable guest posting. Go to your administration panel, then to Forum Admin > Permissions, and change the mode from Public to Registered for all affected forums.
Download the Visual Confirmation for Guests
MOD here. I wrote it, so let me know if it causes any issues. PMs are fine, it isn't as if I get many at all. It will add visual confirmation for guest posting, identical to the visual confirmation seen at the registration page. It doesn't require GD or ImageMagick. Make sure visual confirmation is enabled in your administration panel.
Download the Guest Post Visual Confirmation
MOD, which uses GD or ImageMagick libraries to produce a visual confirmation image.
Disable the posting of links for guests by applying the code found here
It should be noted that this is not
an exploit. If you have guest posting enabled, anybody can post; that's simple functionality, and this is nothing more than somebody taking advantage of a previously unabused feature. All of the options above will prevent automated spamming of your boards.