One really neat anti-spam system for this forum would be forcing the user to solve a very simple lemmings-like puzzle. It's unique and different enough from other anti-spam schemes, that it should hold up well for a long time against bots, and could potentially hold off human-assisted registrations at least for a short while. But probably a little too impractical to implement.
While writing some small game to complete that'd require people to have Flash or Java or something like this enabled, one could do something along the lines of the captcha, but aimed for Lemmings:
Auto-generate a small, trivial (auto-generated kinda implies trivial) level as image with a given (possibly even fixed skillset, like one of each), and ask how many of each skill are at least required to pass this level.
That'd require people to at least know the basics of the Lemmings game, which I assume can be expected from any legitimate person trying to register here.
Of course, levels would have to be generated that it doesn't really matter how far a builder builds or how deep a bomb crater is to solve the level, and that there's only one skill combination that takes the minimum amount of skills. It seems feasible, and an interesting challenge to try to implement.
Of course, there's also forums that require people to post privately why they want to join the forum, and get manually improved by an admin (who can decline if the reason is like "cheap pills" or "starfish necklaces"). Has the downside of more work, possibly frightening away some users (especially lurkers who only come to post after a while, or really contemplate whether anything they post really is worthwhile), and users not immediately being able to post.
EDIT: Alternatively,
reCAPTCHA seems to be pretty strong against bots.
