My advice would be a mixture of: A: A good captcha, which doesn't rely on the typical 'distorted image' setup. Some good questions can work there, as can an an

My advice would be a mixture of: A: A good captcha, which doesn't rely on the typical 'distorted image' setup. Some good questions can work there, as can an anti spam honeypot question and a script to measure how quickly a user fills in the form (to block those who do this inhumanly fast). B: Integration with an spam blacklisting service. Stuff like Stop Forum Spam or what not used to work pretty well for me in the past, though there were a few false positives. Either way, the key is that your sign up process should be as 'unique' as possible, since the majority of spammers are either robots or sweatshop type mass submission setups, and they don't want to put in effort to crack 'bespoke' solutions here.