I've often seen signs saying "do not park between the hours of 8am-5pm (or what ever is appropriate)." I don't see why you couldn't just make your own sign and post it.

(and how rude of them!)

You could also see if your city/town has any ordinances prohibitting the parking of cars in front of mailboxes. If they don't, consider proposing this to your city council.


good luck.