RealmMan;171424
@Dreamstalker: Create a dummy package, something that LOOKS like it came in the mail. Leave it where your neighbor it sure to encounter it. Cover it with something that will stain or irritate skin; or rig it to fall apart when he picks it up, spilling something vile on his shoes. (These suggestions are for entertainment purposes only and are not actually recommended – though they'd be funny as hell to watch.)
Or just have a dummy package and rig up a discreet camera to capture them damaging it so you can report them to the Police.