I know a damn fine idea when I see it, and this, sir, is a damn fine idea. I'd put the metal pole about 4-5' in the ground, fill the whole with quik-crete, and then fill the actual pole with quik-krete.
Screw just stopping them. I'd want to hurt them as much as possible. That way, they won't just stop hitting your box, but mailboxes all together. They'll think twice if they have thier arm broken once.