I think locks are a great idea.

Or putting the stuff that is wasted most often out of reach so they have to ask for it.

Do you buy the juice packs? Try switcing to frozne concentrate and water in a pitcher, or buying a family-size container.

Works out cheaper, in my area anyway.

I am not sure what food they open, and don't eat - is it a pre-packaged sandwich or something?

Try buying the basic ingredients so they have to make it themselves. If they yell about the old stuff, tell them it ran out and there isn't any more.

The only last thing I can think of is having a family sit down meal, which is hard for families on the run, I know.