Try the steps in our guide if your neighbours cat is pooping in your yard. Cleaning with white vinegar, placing citrus peels around the garden, and installing motion activated sprinklers or ultrasonic devices should stop them from doing their business.
If the problem persists, speak to your neighbour about it. It is really their responsibility to ensure their cat isn't littering on anyone else's property, so if after talking to them the issue carries on, you could put in a complaint to your local council.