You do need to try to stop this right away.
Make sure they have adequate protein. If you're just using 16% layer pellets or mash, try giving them some 22% turkey grower or even some 26% gamebird pellets if you can get them.
Make sure they have access to oyster shell at all times.
Let them free range if possible.
Give them other things to do, like a head of cabbage hanging on a rope so its tempting and hard to eat -- this distracts them.
Close off the old nest box/area, and install a completely new, fresh one. Make it much higher off the ground than the original -- preferably about 3-4 feet off the ground. Make it small, just big enough to fit into and lay. You might even want to consider a "roll-off" nest box if it's within your budget. Keep all food well away from the nest box area. Also, get some of those ceramic eggs or put in some plastic eggs with something inside for weight and taped shut, or even some golf balls. It can't hurt, don't know if it really helps.
If nothing else, you may have to just get rid of these and start over.