I don't know about how you would do that, but there are other methods. You could require a signup and email verification (i.e. they sign up with an email, then have to go and activate their account) which might block some bot-type spammers. Maybe require them to also type a keycode made of gifs instead of plain text. By having a signup you would have a master list of those who signed up that you could also blacklist if they became spammers.
Also, like LQ does here, disable posting links until after the 5th post.
Note that I have no idea how to actually implement this stuff.
Just seen it done...