My dog used to eat the cat poops that were outside, and I heard if you put tobasco sause or vinegare on the poops the dogs don't like the taste and won't eat it. I'm not sure if your cat would use the litter box if theres tobasco sause or vinegare on it though. Sorry I can't think of anything else, what we just did for my dog is punished her for it and put the cat's litter box inside the laundry room, chained the door with a chain that we could take of to go in and out of the room, and had it so that when the chain was on there was enogh room for the cat to go through but not the dog.
Hope this helps you out.