First, politely speak to your neighbors and let them know the problems you're having. If that doesn't work, try throwing catnip into the neighbors' yard so they'll hopefully stay close to home. You can use motion activated sprayers which should keep the cats away. If you have an existing fence, there's a cat-proof add-on that can go on the top. There are a number of cat repellents on sale. Some people say fresh, unbrewed coffee grounds keep cats away. I know it's annoying, but please don't hurt the cats. It's not their fault, it's the neighbors actions that are causing the problem.
