Accidental ear piercing alarms at all hours, early morning wake up calls, delivered foodstuffs. Sign them up for weekly pickups from ARC, VFW, DAV, and all the other letter agencies that pick up donations. Trip a breaker and put a lock on the box. Let the air out of two tires, or remove the valve stem. Collect and ferment those grass clippings for a week in a garbage bag, then dump on their porch. There are a lot of free publications that they can sign up for too. Buy a tv b gone, a remote that turns the power off automatically without entering a code. Your evening walks would be more entertaining.
Or do you want to go full asshole?