put up posters urging customers to "close the door" behind them
Do customers read postings in the UK? Because they certainly don't in the US.
They should install some kind of mechanism to automatically close the door when it's not being held open. If the higher-ups don't want to pay for it, he should calculate how many bags of crisps it would take to pay for it, and then say when Steven has been prevented from stealing crisps for X days it will have paid for itself.