Defending against false accusations of cat being a nuisance?

I'd start throwing the tennis BACK at their house, and then perhaps put some delicious cat-inticing food in their yard where they won't see it, so that it attracts every stray cat in the neighborhood and if you're lucky, racoons also. Really though this is all very petty and if they have no evidence of your cats being on their property, there is no action that can be taken against you. You should try to talk to these people as calmly as possible and stop acting like children yourselves.