My uncle in Alabama had such mailbox issues, and he took care of them by welding up a heavy steel liner that just fit inside a standard (large size) mailbox like he uses. It was made of half inch plate, and the services of a machine shop (at a major defense contractor where he worked at the time) were enlisted to make the sucker.
He put it up,and waited.
Kids in that area tend to prefer doing drive-bys with a good hard swing at the box from the window of a moving car.
Ever since he put this monster up, from time to time he finds the shattered remains of a baseball bat or a hockey stick lying near the mailbox. And coincidentally, a few kids have come in to the emergency room with a shattered arm or wrist at about the same time, and of course they always 'fell' or something.
His mailbox hasn't been so much as looked at funny for a couple of years.
And he's a reserve deputy, too. He's taken a cruiser home from time to time and parked it in the driveway as a gentle reminder for the unwary.
Perhaps the funniest part is that my uncle's wife has put those shattered bats and sticks to good use, as stakes for her plants....always staked up out front where they'd be visible to passing cars, especially if the occupants were busy checking out the mailbox.
CJ