I wouldn't call myself an expert on anti-spam measures, but on our site (my own, not this one), we have Glow Host spam-o-matic (which ForceHSS linked to above) and use questions and answers, and we get 5-10 spammer registrations a day. I believe they're actually humans and there's really nothing that can be done about it (assuming you don't want to moderate registrations or block certain countries by ip). In any case it's a trade-off between making it easy for real people and making it hard for spammers.
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