If they're big enough it goes without saying that setting a few basic rules about snack times and meal times should do it (I couldn't overlook the ideal option). Telling them to stop and say or think what they want first, and say what it is (grapes? peaches?) might help cut down the standing-and-looking. Telling them that all the food will go bad, or that the refrigerator will wear out too soon, might help. They ought to be understand that much and maybe work with you on the stop-and-think-first rule because of it.
If they're really little there are refrigerator locks in the child-proofing section of stores. Even if they're not little, you may want to put one on to encourage the stop-and-think-first behavior.
Assuming that won't work, one thing that might help some is to buy snacks (like snack-pak applesauce, fruitcups, etc.) that don't need to be refrigerated before opening. There are those little "juice box" milks that don't, and cereal, peanutbutter, breads, and crackers don't. Neither do individual little cans of juice. If it's water they're after you could try keeping it in something outside the refrigerator, with ice or a cold pack.
Maybe even just buying those kinds of snacks and drinks for awhile would get them out of the habit - and then you go back to buying whatever you buy now.
Other than that, I don't know... Maybe you could do something like set "snack time at 2" and before everyone starts to head off after having their snack, do a check to see if any of them wants to take a juice box or apple with them once "snack time" has been taken care of. That's all I have (that, and maybe baby gates at the entrance of the kitchen doors. :) ) (Or, you could set an "ask me" rule and tell them no more getting their own stuff for now. Have them ask you. That's a pretty simple and memorable rule. Maybe if you did that for awhile it would break the cycle of what's going on now, and you could start "clean" with new rules later.