Blow out a couple of eggs, fill them up with mustard and a bit of food dye. Seal them with wax. Put them in the nest in place of eggs. They don't like the taste and the dye will help show who is doing it. I had to do this for a few days till it finally stopped.
Some people have used the hard ceramic, clay eggs, pick up the eggs more frequently,or cull the egg eater. Check the nesting material so that it is deep enough to not crack eggs. Several hens in one nest will break eggs already layed too.
Seperate the egg eater, give her the mustard eggs to peck at she will stop. Some hens will not eat their own eggs. Search here in forum for other ideas on how others stopped egg eaters. Good luck!