Shoutbox

Why don't you just add an image verification to the PM form, or something similar? That way, the only way a user can send a PM, is if the verification has been done. If the user signs out or something, then tries to access that page (where it would send the PM again), it wouldn't work, as the verification number would have changed. Defeats bots too, as they can't read the image (as long as it isn't just plain text written on a blank background...).