The word 'booby' on its own could mean one of several things. It could be the name of the server, although it's a very unusual name if so. It could also be the name of the restaurant, but that doesn't seem particularly likely either.
What we really hope is that it's not a reference to the appearance of the customer who paid the bill, because that would be horribly out of line! We'll never get to the bottom of this mystery, but it's probably not a word you should habitually print on receipts.