you are very right, people must follow rules
@MB Themes
you can limit user referral for 2 or 3 times by ip
more idea:
Do you have any idea to find the referral registration on same computer except ip & cookie?
let's say :
Whatever referral link he clicks, the php code will carry forward the field data of referral code in the link, and will keep track of it until he gets to the registration page.