Mention that you keep no records. If you are getting back at somebody with a multi-line system, this is good. Place a large magnet where the line comes in. When one phone rings, they all do If you can get a private minute with your marks phone, and the handset is modular, cover the handset plug with clear nail polish. The door will work fine, for a while, then fall off.
People start screaming.
Add luminescent paint into the cans of someone who is painting their fence. Then, at night, it glows. Get some copper paint and paint a small line across the insulator of a spark plug in a car. They'll never find it. If your loser gets a parking ticket, get it before he sees it.