How can I prevent more than one purchase of a single item?

It is possible to prevent more than one purchase of a single item.

One method is you can enter the Qty in Stock of the item to "1" while setting
Site Settings > General Settings > Hide Products If Not In Stock to Yes.

However, this is not very efficient, as it would need to be manually reset after each purchase.

A second method is:

1. Go to Products > Product Catalog Manager.
   
   
2. Click the Edit link beside the product for which you would like to limit purchase to a single item.
   
   
3. In the Simple tab of the Product Editor, select the checkbox labeled Hide Qty.
     
   Note: With this setting enabled, customers will not see the Qty box on the storefront and will only be able to order the number defined in the Min Qty box of the Product Editor.
   
   
4. Ensure that the Min Qty is set to 1 (the default value). With this set as 1, and the Qty box on the storefront hidden, customers will only be able to add a single item at a time to their shopping cart.
   
    
5. Click the Update button.
   
   This will hide the Order Qty box from the product details and the shopping cart pages for this product.
   
   Note: The setting Hide Qty? is actually a display setting which hides the order Qty text box from the Product Details and Shopping Cart pages. It does not disable it or prevent multiple Qty from being purchased through buy-button snippet codes. In other words, multiple Qty of the product can be added to the cart if your buy button snippet code is set to submit a Qty greater than 1 or if the customer goes back and presses the buy button multiple times.