在匿名用户可以投票的网络竞赛中，有没有可靠的方法可以防止作弊?

I'm working on a web-based contest which is supposed to allow anonymous users to vote, but we want to prevent them from voting more than once. IP based limits can be bypassed with anonymous proxies, users can clear cookies, etc. It's possible to use a Silverlight application, which would have access to isolated storage, but users can still clear that.I'm working on a web-based contest which is sup