What to do? My friend's neighbors have been taunting them and making their lives totally miserable.?

They have been hanging things on their fence, piling garbage on the side of the fence where their back yard is. They are very noisy and nosey, they have made it very clear they don't like them they stole their last recycling bins and bought new ones twice now this time they spray painted them to be sure no one would take them. Well they took them again....What to do about unruly neighbors? My friends refusue to sell their house.