What forum software is it?
Stupidly unique registration questions (like the Mercenary one here) tend to help more than captcha does, since the bots have long been able to get around it.
The main reason though really is the mod team. At any given time, if you check the last few registered users here the most recent visitor to their user profile page is almost always one of us. Beard is the champ at it - he's like a new member checking ninja. If the profile is bullshit we just ban it.
Occasionally we ban a legitimate user with that policy (and I get hate mail) but the reg process here plainly states "Hey, fill this stupid shit out or we'll ban you". It's not that I give a fuck what anyone's favorite color is, I just want to know that the person registering knows how to read.
I also have a server level block on the entire Chinese IP address range, though that's more to try to prevent the hacks/brute forcers/etc than the spammers. It catches a good deal of both. I still get a fair amount of alert email about it, but it's gone way down since I made the change.