Since our inception of the visual antibot and the question antibot plugin on AMV-Canada’s phpBB board, we’ve completely eliminated the bot problem. Quite simply, since May 4th 2009, we haven’t had ANY bot come through. We’ve even reduced the complexity of the captcha, and eventually disabled it completely, but to no avail, spammers haven’t come back.

Yes, I mentionned the visual antibot, basically an upgrade captcha almost impossible to figure out even for human (puts random pictures in the background, making it REALLY hard to read, we’ve actually had some people complain to us that they couldn’t legitimately register), but we got rid of that. The only thing we have to protect ourselves on top of the default installation is this: The Question Antibot

That thing is holy. Basically it’s question you make up and provide the answer for. Example, a mathematical question. It’s also super easy to set up and change everyday if you want. One big advantage is it putts off just about any bot, because they don’t know what to do with it. And if you have a large traffic site and some body programs the bot to answer the question, just change the question! It’s so efficient you don’t need a captcha.

Unless computers become sentient, I believe this should put off just about any spam bots.
Hit the link: http://www.phpbb.com/community/viewtopic.php?f=69&t=645075&start=0