1. A referral commission in percent of what the referral earned. If you earn eg 3.90BTC from them through ads and pay out a 10% commission of 0.39BTC, you still keep 3.51BTC. Then there is abosutely no reason to cap the commission, unless you want to piss of your partners by looking greedy.
2. A "bounty" program where you pay for every referral who signed up under a user. If you do that blindly just based on sign-up numbers, well no surprise that it gets abused. It's easy to create hundreds of fake accounts and pile up the bounties on the main account. Tie that to a requirement, eg "bounty is only paid after the referral earned amount X".