removing quantity support for products

The store that I'm currently working on is digital products only - I have disabled the 'show quantity' field in the store, but when a user clicks 'add to cart' for a product, it still adds to the quantity of the product in their cart.

Basically I want to remove the ability for users to have more than a single entry of a particular product in their cart - is this possible in marketpress or am I going to have to go hacking?