All things aside, that mailbox is now his territory. I suggest you do what is necessary and piss on it. Otherwise you're mailman won't know where to deliver you're mail. Do it, and do it meow!
Step one: get a squirt gun (one with decent range)
Step two: piss in it until it's full
Step three: wait for your neighbor
Step four: hit him over the head with a bat (this step is optional)
Step five: squirt him with piss
(if you did step four, make sure he is conscious before proceeding with step five)
It would be easy to get away with in the country. Though if we saw you and you were not a drunken friend we would not bother with police. We would take it into our own hands.
Call the cops, say he was naked and he'll get a sexual predator sticker on him. He shouldn't have fucked with you or your mailbox.
That's a double whammy.
That's fucking with a piece of Federal property. What a dumbass. ^_^