I have my own server but same thing should work with Parse I guess. Basically the client has to check with the server if the user has ever been referred before. Also after choosing the referrer I hide that option on the client.
I also put a limit on how many people each user can refer so that if they decide to fabricate these users to get referral bonuses they can’t do it forever.
This isn’t live yet so I have no idea how successful it will be but it looks like this 
