Disable shops for a player?

What I would like to do is be able to disable the ability to purchase items from certain shops once the player has purchased a certain quantity.

For example, after a hero has purchased 4 items from this shop, I need to stop him from purchasing more. But other players need to be able to purchase from it until they each reach 4 items.

Oh, and the items purchased from these shops won't remain in inventory.

Any ideas?

Thanks,
Trick

React on the event that they buy from that shop and count the number. Then you can give another order when they are actually not allowed any more to buy.