I would put up a nice solid fence on YOUR side of the property line. And then paint their side hot pink with lime green stripes. If they paint over it, hire a professional painter to fix it and send them the bill.
When I put up my electric fence I set it back at least a foot. Only way to touch it would be to be trespassing on my property. If I was really hateful, I would plant some of those ugly vines that stink and then put a camera on them.....so when they try to kill or cut them, hire another expensive person to fix it and send them the bill.
Then take them to court when they dont pay, with video as proof.
It would be worth it for the satisfaction.
Also, their cat would disappear permanently next.