My chickens have no problems eating mustard, or hot peppers or just about everything else folks suggest putting inside eggs to discourage a chicken from eating them.
Removing the offender, adding more bedding, gathering often and making the nest boxes darker are all excellent suggestion though.
But, another thought. Be sure your chickens are actually doing the deed. I regularly feed my chickens eggs, usually the soiled ones. I chuck them into the run so they crack open. You know what's left behind? Not a darn thing, they eat it all, including the shell.
The last time I found eggshells laying around in my coop, I had a serious mouse problem. You might want to start setting some mouse traps at night and see if the problem doesn't resolve itself....