Sorry, to hear that Skeat!
We had a neighbor that used to come home drunk all the time and when backing up his car to make the swing into the garage he kept hitting and knocking down the posts in my chain link fence.
After he paid for the repairs several times I decided to fill the posts with cement...lets just say, his car lost the next time.....
:D
I like the idea of the cement blocks, fill them with cement, then use a mount, like for a deck post, into the cement and attach the mailbox....it WILL HURT when they hit it!