All great suggestions. May I add, wait a week or two until they think they've gotten away with the crime. Then, hit 'em.
Buy an inflatable dummy, or rent one from the DUmp, knock it around some, maybe spread some ketchup all over the "body", leave it on their front lawn in the wee hours, then have an accomplice call in an anonymous report of a dead body at your neighbor's.
Don't forget to wipe any fingerprints off the dummy.