Many do like the mustard, birds don't have capsaicin type receptors.Try mustard eggs.
Hollow out an egg by poking a hole in each end, and blowing the insides out. Then take piping bag and fill it with mustard. Put these eggs in the nesting box, so when they break them open they get a nasty surprise. Most hens don't like mustard eggs. My hens liked the mustard lol! I have egg eaters to.
Use liquid dish soap if you're going to go to the trouble of filling and egg with something.