Event: Food used of (player) becomes equal to 0
I'm not sure that event exists, though I know there is an event that triggers when a real number becomes a certain value or range of values, and only a real number. I don't think food used is a real number. Otherwise, do this.
Event: Every 2.00 seconds of the game.
Condition: (Integer Comparison) Food used of (player) equal to 0
Edit: don't forget to make the creeps have cost at least 1 food.