Try crushing the mothballs, use the old fashioned kind ( naphthalene) with a hammer and sprinkle it out lightly, if it rains you will have to reapply. It worked on my cats on my lily beds in the past. I now just keep a bare dirt area fluffed up and clean for them. putting a litter box outside has never worked, it seems given a choice they do not want to use a litter box if anywhere else is available. All the cats in the area never caused the amount of damage one dog can do. Even when the killer cat would bring her prey home to turn out in the lily beds to chase and re catch.
