How would you deal with these neighbours?

What you could do is call the police about the fence thing.If you want to avoid the law What you could do is build a gigantic fence to avoid them. also just to be annoying during the fall blow all your leafs over to their side of the fence. During winters you could block their cars in with snow. As for your ball if it goes over the fence again just go in their yard without asking. It is your ball