couple thoughts:
- are you sure it's the chickens, and not a gopher/chipmunk, etc.?
- if it is the chickens, purchase a bushing to narrow the size of the opening, or put something over the hole that you can make a specific sized opening. if they cannot get their feet in it, they can't scratch it out. if they are doing it with their beaks, then this will also not allow them to scratch it out.
- is there anything else in your pen that they can "play" with/in? for instance, throw a slab of straw/hay in there for them to scratch and pick through; might help with boredom, if that is what is driving them to play in your food