...there is no good way to tell real subscribers apart from bots.
Hey, James - I've had really good luck with controlling 'bot registrations by adding a question on the registration form - something that only a person would know, maybe only a person interested in the forum's topic: e.g. what's the ugen to create a sine wave? => SinOsc, or something like that. We went from about 10 bogus registration attempts per day to zero over the last several weeks.
Maybe you've decided this forum is redundant to the very active sc users newsgroup, but if not, there seem to be some mods available for your forum software which provide the 'registration question' feature:
http://custom.simplemachines.org/mods/index.php?action=search;type=13