My kids do the same thing. I just don't buy that item again for a while. I will bring the half-eaten Lunchable or the can of Pepsi with three sips taken out of it out of their rooms, show it to them, and say "No more Pepsi for a month." I even keep a list, that clearly shows food that we no longer buy.

I am, officially, "the meanest mom in the world", BTW ... but it works. Well, it works to some degree. They relapse again, and again. But it's better than nothing.