This is from a talk that Matt Cutts (Google) gave at Web 2.0 Expo , which was held last week in San Francisco.
From the 3:40 mark:
Look for ways to build a little bit of reputation into your system…. You don’t just have to cost spammers money. You can also cost them time. You can cost them effort. If a spammer gets frustrated whenever he tries to spam your system and get a reward, he’s more likely to bypass you and go and look for some easier prey. So think about the spammer mindset and figure out how you can annoy them.
