My kids always eat constantly through a school break and I decided that I was going to try something new. I put containers in the fridge with their names on them and had each child pick out 4 snacks in the morning. These were the only snacks they could have throughout the day. They didn't need to ask permission to eat any of them, but once they were gone, they were gone!
It took only one time for my son to learn his lesson. He decided to eat all of his snacks in the morning and was hungry in the afternoon (right before dinner). I didn't cave in and he never did it again. It was a lesson and although I really was tempted to give in, I knew that i would regret it. It's ok to say NO...They aren't going to STARVE...even if they say they are going to (:
Hope this helps you to save some food, money and a little sanity!