You could in principle very easily ignore the dice and eat the chocolate regardless. You need to take it upon yourself to follow through with the scheme and forfeit the chocolate 3 times out of 4. If you start with the understanding that chocolate is a possibility 4 times out of 4 if you followed a more permissive scheme, then you are effectively punishing yourself 3/4 of the time, which I expect would work as negative reinforcement for said task or the reward scheme in general. And it would also require enough willpower, which some people won't have.