Spam sucks. Any site which allows unauthenticated users to submit forms will have a problem with spamming software (spambots) submitting junk content. A common technique to prevent spambots is CAPTCHA, which requires people to perform a task (usually noisy text recognition) that would be extremely difficult to do with software. But CAPTCHAs annoy users, and are becoming more difficult even for people to get right.Rather than stopping bots by having people identify themselves, we can stop the bo...