You could try laying down chicken wire just under the mulch. Or even on top of it. The cats don't like getting their claws hooked on it.

Try cayenne pepper also. But you'd have to do it every time it rains. My DH pours concrete and has a sand pile, and every cat in town knows where it's at! He's resorted to making a chicken wire hat, seems to keep them from the pile, tho they do poop on the ground NEXT to the pile.