You'll never prevent abuse fully. People will always find a way... garbage in, garbage out. We're flawed, so code is manipulatable. My suggestion for a referral reward would be to mark the referrer as a 5% beneficiary with a time lock of 3 months. I would love to have a way where I could give someone a referral link where when they sign up, I automatically delegate 25-100HP to them and it simultaneously locks me in as beneficiary of a percentage with both locked for 1 month-3 months. This would benefit them to get my delegation where I can have a one time use referral link to prevent abuse of my delegations and the lock would cause me to be rewarded for referring them in the first place. Just some thoughts...