Identifying a user behind a proxy.

[tobes]

Hi guys. I have a little bit of a problem. I'm a design partner in a fairly popular Web Site in which people login with an ID number and refer new users for points. When a new user logs in for the first time, the person who referred them earns a set amount of points. This is how users are arranged. The more points you have, the closer to the top you appear.

We've taken a few measures to prevent cheating such as keeping a cache of IPs and only awarding referral points if the ID originates from an IP that's unique to the cache. Of course, an obvious way around that is to consistently hide behind different anonymous/elite proxies and refer random IDs from your own machine.

I know of other sites with themes similar to ours that prevent this method of cheating. Does anyone have any suggestions? I guess what I'm trying to ask is if it's possible to uniquely identify a user's computer even if they've decided to hide their IP behind a proxy?

Any help is appreciated.

Thanks.