Are you human? (or How to prevent spam) [closed]

Answer 1

From xkcd

Answer 2

This is the study area of Human Computation.

there is an excellent video from Luis von Ahn here: http://video.google.com/videoplay?docid=-8246463980976635143

Answer 3

From Quantum Random Bit Generator Service, via MNeylon

Answer 4

• Limit the number of votes per IP address per time
• Block anonymizing proxies.
• For voting: How about shuffling the value that has to be returned by the form on a "per session basis". Once "1" means the first item, "2" means the second. Then "77" means the first item, "812" means the second, ... could be some simple maths behind the scene, but it prevents users from just sending the same HTTP query over and over again.
• What's worked for me very well: Use AJAX forms, not simple HTTP forms. Technically it's not much more complicated to fake votes, but I have written a simple blog software and it's only SPAM protection mechanism is to submit the comments via AJAX - no SPAM so far.

Answer 5

I normally use a combination of the two: anonmous user is free to browse everything, but if he wants to vote, then he has to register.

In the registration process, depending on the situation, I use an optin thru mail (to complete registration and confirm that at least the mailbox exists) and/or a CAPTCHA.

From that point on you can decide if the user can vonte more than once, or any other rule.

Btw I'm not a fan of the IP-based constraints: there are a lot of situation in which big organization's network use few IP for all their users, so the risk to block users that could vote is high.

Answer 6

For a CAPTCHA system, I heartily recommend reCAPTCHA.

Traditional computer-generated CAPTCHAs will eventually be broken by developing a sufficiently intelligent system. For instance, here's someone who claims to break the Google CAPTCHA, formerly considered unbreakable, with a 30% hit rate. reCAPTCHA, by definition, shows you only images that cannot be recognized by optical character recognition.

And at the same time, your users' effort will be directed towards the common good - they help digitize books by recognizing words that cannot be recognized automatically.

See here for further explanation and to try it out.