You might want to do this to reduce the chances of overselling an item.

In EZ Inventory, you can set this in the Data Feed settings under the Field Mapping section.  There's an option for "Use a formula to determine quantity" where you can enter a formula like below:

0 if {{ Quantity }} < 5 else {{ Quantity }}

The formula above will use the quantity of 0 if the value of the "Quantity" field/column in your inventory feed is less than 5.  Otherwise, the original quantity shown in the inventory feed will be used.