If you operate a business in the Internet age and delude yourself that customers are required to have decorum, you are in for a rude shock, sooner than later.
Just look at Ham Radio Deluxe.
If you've already sold them the product, you really cannot do this.
The store owner could go to jail, because that is called theft.
Screwing over customers has a cost.
This story went viral. And now this business is going to receive hundreds of negative reviews.
As it stands, this is how ANY court of law would see it as well.