Open Source Referral Program - Preventing Fraud

[jpollar]

Anyone have any experience / idea how to build a multiple tier referral program using PHP/MySQL that also prevents fraudulent referrals?

I've found scripts that will do the referrals...

They just don't stop the same person for signing up for multiple accounts and referring their own accounts.

I can't use IP Addresses because users could just get on another machine...

Any one have an idea how sites like http://www.linkbucks.com prevent referral fraud?

[jpollar]

No ideas at all?