Look for dried egg yolk on the feathers around their beak. That will tell you who the culprit is. You can isolate that hen out if you want. Then the egg eater won't be teaching all the others the new trick.
Then supplement their diet with a source of calcium (if you haven't already). Oyster shell or dried/crushed up eggs shells are both good sources. It the shells are thick it will be harder for them to peck open the egg. Often they learn to eat eggs when the shells are so thin they break when they lay the egg.
Collect the eggs as soon as they lay them if you can. This will reduce the time they have a chance to eat them.
Last resort for a hen that won't quit eating eggs is to eat her.