Organizing a contest of bringing referrals to the forum and giving monetary reward to people with most referrals.
I can think of 56 different ways to abuse this.
That can actually be a good thing. If the admin decides to implement this, message the mod those 56 possible abuses and he will find out the security measures to avoid them.
Are you going to keep asking admin to install anti-cheating strategies on every revealed situation because LoyceV's 56 conceivable ways to defeat this system may differ from my own 60 ways of cheating the system? The truth is that those who wish to study or read about projects must make their way here one way or another without anyone telling them to; in fact, the majority of us found our way here without a referral or word from friends.
There's a good possibility that anyone interested in cryptocurrency will do some research on Satoshi, which will eventually bring you to Bitcointalk, where he once discussed bitcoin development.
There are ways people try to game the referral system in all sites. No system is 100 ℅ perfect. But many great companies have grown with the power of referral like PayPal.
We already have a large number of inactive accounts; out of the 3 million or so accounts created on Bitcoin, the active accounts number fewer than 500k (saw that stats somewhere can't remember.). The referral problem would be far worse than bounty spam..