Ask HN: How do you prevent people in a referral system from referring itself?

I have a referral system in my website where I award users for referring people to join my website. I know that IP address can be faked so that can't be used in my use case. Letting the users enter mobile number is to extensive for my system and entering credit card is also very extensive for my system. So is there a service who can track referrers uniquely? Or how can I build one?