Obviously, distortion text, which this forum is using IIRC, is hard to crack, but it has already been cracked by someone, and spambots happily use the crack on any forum they find. Even asking a very easy non-standard question should work, like "Which one is older, ADOM or JADE?". A bot can't answer such a question if it has never seen it before (unless it asks for human help).
(Note that I *think* it would work, and it works on my small site, but I am not completely sure. Maybe these bots are able to ask humans, and/or have very good AI.)