get a scarecrow water jet, they work, cats only get hit once and they know not to come back 2 of by next dor neighbours have 2 cats each the other 1, 5 cats next to me but they dont come in my back garden, although i havent had my water jet up for a while i must rig it up again just in case, i used to set it at night and you could always tell if it had been set off the back yard was vet wet, and they are legal. no use trying any harm as you will only get into trouble and after all they are someones pets, dont like them myself wouldnt have one in the house, but we have pigeons some dont like but would we like them to harm our birds,