Unfortunately, that is not part of the current phpBB package we are now using. Not sure if the new version will have it or not.
That would not be easy to program ourselves, I'm quite sure.
Also, the visually impaired are not all that happy with such requirements either.
One possible alternative (such as is used for those making comments on the manual at http://php.net
) is to ask a simple question, like "what is two plus six" or, even less hackable, "what is another word for the day after today".
This spamming is yet another reason pointing to the need for:
1) A representative global government (maybe they will make a fixed law against such spam)
2) The Bahá'í Faith (and moral education with the influence of religion). No matter how clever we are in defending against spam, the war will only escalate with more sophisticated spamming techniques (of course, we nevertheless need to take action against it in the meantime) as long as society has such a limited interest in moral development.