Hunting cheaters in a voting competition

Question

Currently we are running a competition which proceeds very well. Unfortunately we have all those cheaters back in business who are running scripts which automatically vote for their entries. We already saw some cheaters by looking at the database entries by hand - 5 Star ratings with same browser exactly all 70 minutes for example. Now as the userbase grows up it gets harder and harder to identify them.

What we do until now:

1. We store the IP and the browser and block that combination to a one hour timeframe. Cookies won't help against these guys.
2. We are also using a Captcha, which has been broken

Does anyone know how we could find patterns in our database with a PHP script or how we could block them more efficiently?

Any help would be very appreciated...

Answer 1

I use a combination of CAPTCHA, IP verification and LSO (Flash Local Shared Objects, hard to find and delete for common people).