The best idea is to get some Japanese knotweed. Have every neighbor plant it and then put their properties up for sale. People have committed suicide over that being so destructive to property. Once discovered, no insurance company will touch him. By the the time he figures it out, that hell of a nightmare plant will have begun to grow up his soul. Honestly, people could do that with all investor properties, or those of other bad citizens. War is upon us.

Also, arsenic, lead, radioactive material, poison ivy, poison sumac, ragweed, poison oak, hornet, bees, bajillions of mice. There are an infinite amount of things that can be done to fight the bigger pest.

They could also use ultrasonic pulses through compression drivers pointed at his house constantly and see if he goes nuts. I'd have more fun from the headphone threat than if he just left me alone personally. Hell, I'd trade houses with one of these folks just for that entertainment.