I've dealt w/egg eating chickens several times. Farming for Fun is right-- mustard does the trick! I blow out an egg--use a pin to prick holes in both ends--make the holes a bit larger.... I fill a syringe (bought at local feed and grain) w/YELLOW mustard--I can't afford dijon--fill the egg full full full w/mustard--and I put some athletic adhesive tape, or a band-aid piece over the holes so the mustard stays in. Wipe the egg off so it looks perfectly normal.....Then put it back in the nest box. I must say, there's nothing quite as satisfying as seeing the expression on the hen's face after she gets a mouthful of mustard. Priceless!
I've been told by others/read on this forum that egg eating is sometimes caused by low protein--you may want to supplement your girls w/some suet, mealworm treats, or add some grower (non medicated) to their laying mash. I buy suet in 5 to 10 lb chunks from the local butcher, put it thru my grinder, and the hens go crazy--especially now in late winter when they're not getting bugs or worms.
You will probably have to repeat the mustard egg trick several times... I had an *addicted* egg eater and it took 3 mustard eggs for her to quit. Much easier on ME than wringing her neck....
Good luck!