There's a supplement you can buy that appeals to dogs but makes their poop unappealing to dogs that like to eat poop, including their own. Must be available for cats too.
It worked perfectly for my long-gone rat terrier. Sorry, I do not remember the product name.
Perhaps hitting the litter with anti-chew spray (do they have that for cats?) would help, but do not overuse it and drive the cat away from using the litter box.