Once chickens have got into taste with eggs it's really difficult to teach them otherwise.
Other than getting the eggs from nest first..or literally taking them as they 'drop' out, there is not much else to do. All the mustard tricks etc. are useless.
I keep couple of golf balls in each nest box all the time..sometimes they get odd egg with thin shell and those break easily and often can start the bad habit. My theory with golf balls are that they are preventative rather than cure. When chickens lay in nest they weight is on the golf balls and not on weak egg that break easily..

So far it's been working for me..