Only one answer? Wow.
I'm not sure what you've tried already, but try an actual PADLOCK on the fridge with a key or a 4-digit code type lock. My parents did this with their liquor cabinet when we were in high school. Here's a picture of one they sell. Found it googling "padlock on fridge." http://homegarden.productdealsonline.com/wp-content/uploa...
I agree that you should NOT have to go to the store daily. Your kids need to be under control of their behavior - not you being controlled BY THEM to go to the store daily.
Can you close the door to the kitchen and lock it? Can you be consistent with their consequences when they disobey your rules regarding the food - automatic time out, automatic spanking, automatic consequence of some sort. Can you give them snacks during the day? Good way to teach them time -- "Breakfast is right now at 8:00. We'll have a snack when the clock says 10:00. Around 12:30 we'll eat lunch....etc." My last suggestion is to make sure your boys are getting lots of good protein, esp for breakfast (scrambled eggs, bacon, hardboiled eggs, french toast, ham, yogurt...). Protein will keep them fuller for longer.
My 2.5 yr old son wouldn't get away with something like this - he would learn very quickly that he will get in trouble for disobeying the rules. Remember when correcting and scolding and punishing your kids, make sure they remember WHY their being punished. Remember to get down to their eye level and make eye contact with them.
Good luck!