This does not seem to work at keeping out the spammers. I think only allowing Moderated posts for a newbies first few posts, and permanent banning of Spammer violators will work (those persistent enough to learn enough to pass the real vs spammer test), and that might not work.
In a forum, spammers are the electronic equivalent of having gang bangers crash your daughter's 11th birthday party. Nothing wrong with having bouncers. Might need more bouncers.