Best one I ever saw was somebody took one of those oversized post hole digger drills, you know the size that the utility company mounts on it's trucks to drill holes for phone poles and electric poles.

Person found one somewhere and put in about a 4' footer and put that drill into solid concrete with the mailbox on top. ... oh and he painted that sucker in reflective paint

SCAREY looking sucker that you KNEW was going to TOTAL your car if you hit it!!

After he did that his mailbox didn't get hit again at least while I was around, though it had been taken out a few times before that, so it must have worked for awhile.