From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-1929-860603
Question: How can I keep cats from digging in my flower pots?
Answer: Cats can be real pests digging in flower pots, as can
squirrels, chipmunks, and even toads. Here are some things to try:
Keep the cat indoors; cover the soil surface with gravel, large mulch
chunks, thorny branch trimmings, or chicken wire; or use a repellent
spray such as the commercial formulations or a homemade garlic spray.
There are also motion-activated water sprinklers available.