Acquire some golf balls from a charity shop, paint (or handle with muddy hands) to stop them being too white, drop one into each nest box and scatter any extras about the place. Sometimes this can fool them in to thinking they are impenetrable eggs & they give up trying. To do a more realistic job, you can full eggshells with plaster filler which means you need to sandwich the 'lid' on, so leave space for this. As you can tell - been there, done that, and it's much cheaper than buying dummy eggs...........particularly if fox, squirrel, rat or crow makes off with them! I don't think chicken have much sense of taste - mine used to enjoy left over curry.
Guinea fowl & a lot of surplus poultry equipment.