Moving the reward into a currency is not a difficult transition. The reward begins as a function-able token, It has a purpose it may be used in some manner of redemption for an alternative reward. It is basically a currency with 0 value outside of the circle it operates within. Making it a currency is as simple as giving it a value.

In any system that is developed and implemented there will always be a % which will see a method to gain some advantage. Every system has a loophole which can be manipulated. Every reward system holds its flaws. The only method to control this abuse is to track each reward and have it only redeemable by the identity it was issued to. This results in a two way trade only. and the token cannot be traded to third parties. This will not isolate the loopholes or abuse of the system. it will merely lead to other already established methods of gaming the system being used.

Just a thought.