Definitely avoid moth balls and I can tell you from experience that pebbles on top of the soil will work plus the pebbles help keep moisture in the soil so it's a win win solution ;-)
I buy pebbles from the Dollar Tree & put in my houseplants, they look pretty & the cat doesn't try to dig in them. Also it doesn't effect watering your plants.
I would avoid the mothballs. They are poisonous. I put a layer of river rock on the soil of my houseplants. The cats completely lost interest.
When I had house cats I used to put moth balls in the plants to prevent them from digging. You could probably put some rocks in there too, then they wouldn't be able to dig.
Add your voice to the conversation. Click here to answer this question.