even if they're not eating eggs to get calcium, if the eggshells are weak (from insufficient calcium) that can give them Ideas. Oh, look, this funny thing that came out of my bottom broke open, I wonder what the inside tastes like.
If they're confirmed in the habit now, and/or if the eggshells are already very strong, calcium may not have anything to do with it... but for others, who are just experiencing the very beginning of egg eating where the eggs are rather fragile shelled to begin with... for them, I think it IS worthwhile making sure they've got free access to extra calcium sources. (Well, which they should probably have anyways, but you know what I mean).
Try tossing them some meat scraps. Probably also can't hurt to put a couple of egg-shaped and -sized rocks in the nest boxes, in hopes they will bonk their nosie and become less enthusiastic about it... although your hens may be past that point by now.
Good luck,
Pat