Thanks for writing. There are a number of repellents you can try; many gardeners have success with these. Or, if you're home when the cats invade, you can try spraying them with water from your hose. Most cats learn after being doused with water two or three times to stay away. You can also try putting decorative fencing around the area; that may deter the cats and cause them to go elsewhere. Otherwise, if your area has leash laws for cats, you can ask your neighbor to keep the cats in bounds or otherwise you'll be forced to contact your community's animal control department.
Note: Always wear gloves when you're in the soil and wash your hands thoroughly afterward. Cat waste can carry diseases that can make you sick.
---Justin, Senior Garden Editor, BHG.com