give him a credit note
simple but fair
you dno't lose his "custom" but everytime he rings up asking to use it say "sorry, i don't have that in stock at the moment"
he'll soon get tired and when he wants to buy something a lot more expsensive and return that one too, again, offer him a credit note

every scrapyard i know does this, you are running a business with the same sort or get up so you've got to play by the same rules