Firstly, let admin enable/disable referral for users instead of allowing all users to have it..this would prevent abuse and fraud because you will be able to selectively allow whose people you know to access it. Secondly, you can add Powered by at footer of the vcards and if the user has referral enabled his powered by could automatically link to his referral code. Thirdly, allow admin to set frequency of referral earnings for example options like one off commission, lifetime commission, first x months commission.