Cat faeces is extremely desirable to a dog because it contains much of the same allure as cat food. If you have a dog that decides cat faeces is an acceptable treat, you cannot easily train that dog to cease this behaviour. Raiding the cat box is rewarded by the cat faeces your dog finds to eat. Even if you punish your dog, the reward of the cat faeces will probably out weigh the drawbacks of any punishment. Avoid the frustration of a cat box battle by removing the temptation of a cat box full of faeces.
Keep your cat box well scooped.
Move your cat box to a room or closet that is not easily accessible by your dog.
Place your cat box on a shelf or dresser at a height the dog cannot reach.
Cover the cat box and position its opening so that the cat can still enter, but the dog cannot.
Install a cat door on the door to the room containing the cat's litter box to prevent your dog from entering the room but allowing your cat to still have access. Baby gates at doorways will also prevent a dog from entering while allowing the cat free rein. If your cat cannot jump over a baby gate, raise the gate up a little so he or she can scoot under it. A small dog may be able to manoeuvre through the same openings as a cat but will have more difficulty reaching a high spot. A large dog cannot get through small openings, but he or she may be inclined to jump up and knock a cat box off of a shelf or dresser. Make sure your cat is OK with the new litter box situation. For example, some cats do not like covered boxes or cannot jump to a high litter box position. You may have to try different approaches to find a solution your cat is comfortable with and to prevent your cat from deciding not to use her litter box anymore.