Heidi,
Big gravel is good, so is fine chickenwire, also foil.
I have many house plants. I make sure I have no toxic ones or grass like ones in case someone decides to take a bite.
A squirt bottle with plain water is good if you happen to see him digging too.
My cats hate foil so if they claw or go where they shouldn't that stops them.
I would not use mothballs because they can be toxic to pets and people.
A layer of coarse orchid bark,potting media, that you get at Home Depot or Lowe's might work too.
Let us know what happens.