I prefer to bake the eggshells from a few days' worth of breakfast for a little while in an oven. Then, I crush them into little bits so they are not recognizable to the chickens as eggshells.I have had egg eaters and stopped it a few times.
Mostly when lacking protein the egg becomes essential. Scramble some eggs for them, probably 1 egg for 4 hens is enough. Once or twice a week. Let the shells dry a day or 2. Crunch the shells and give as calcium supplement. 1 shell 2 or 3 times a week.
Overall the best solution is a complete feed, preferably from a feed mill. They can still free range. But these other ideas will help for now.
If oyster shell were available, I would get that, but there's no oyster shell in my neck of the woods.