I usually erase 3-6 spam threads per day, that's really too much. I don't remember, is there a Captcha system when signing in? It would be useful.
Also, Giacomo's idea is good, but maybe it could discourage some. I don't know if I would've registered if I had to "pass a test", I'd feel it as kind of smuggy

But having a moderator or an admin approve the new user is a common practice, and it's not too fastidious for users, nor too hard to do for us mods. That way we could not accept those with obvious spam names, like "iphonecheap1Hj2", and accept only if profile is at least half full.