How often to reward "leave it"?

I am at a total loss, my puppy is a very quick learner but also knows how to play the system. He learned the "leave it" command in training exceptionally well and started applying it to other things, however he now has worked out that if he goes over to something he shouldn't have and then leaves it when i tell him to he gets rewarded, but then he goes back to it and back to it and back to it so as to be rewarded for leaving it multiple times. and if i don't reward him then he knows i'm out of rewards so he just get the thing he's not supposed to have. The only rewards he cares about is food because no amount of play, toys, praise or cuddles seems worth it to him. Please help I am stuck and stressed