A mean way to get revenge on someone is to purify poison ivy and smear it on door handles in their house that they rarely use. The substance is a clear oil, and it always takes 4-72 hours from exposure before a rash appears, so they're unlikely to trace the recurring flare-ups to you. For more details, see US patent# 9580373 ("Extraction and purification of urushiol from botanical sources"). Just check first that they aren't known to have severe reactions.