B2 has a plugin/hack which only enables commenters to post only through the comments form and not directly through the b2comments.post.php. (wp-comments-post.php for WP). As in, any direct requests to b2comments.post.php is denied.
More info:
http://cafelog.com/board/viewtopic.php?t=5891&start=0
Does WordPress have a similar hack?
