Protecting against fraudulent reviews

[directone]

What's the best way to get some medium level security for my web site's product reviews. What I want to protect against is someone logging in from one computer with multiple user names and writing lots of rave reviews about their own product.

I know a cookie can help, but don't most people disable or clean cookies these days?

Anyone know how amazon et.el. do this kind of thing?

What about eBay's shill bidding algo. Is this in the same field.

Looking for some jump off points to get my research going. Even clues on the right keywords to search for would be great.
Any help greatly appreciated.