Disable quantity off WP-ecommerce

Hi

I setted up Store Settings / Admin Settings Max downloads per file:1, but every time the user clicks on the Add product button, the costumer is able to add the same product infinite times, in this case they would be buying the same image of the same size… so I need to disable that function so it only allows one image of every size.

I already deleted the code from:
<!– THIS IS THE QUANTITY OPTION MUST BE ENABLED FROM ADMIN SETTINGS –>

To: <!–close wpsc_quantity_update–> </fieldset>

on the wpsc-products_page.php file and the code between the following tags:

* Quantity options – MUST be enabled in Admin Settings
<!–close wpsc_quantity_update–> </fieldset>

on the wpsc-single_product.php

But havent been able to block or limit the times the user can order the same picture.

I’d really appreciate if anyone can help me,
– Thanks in advance!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

– JanikMaC