AllProWebTools has several settings that allow you to control what happens when your inventory runs out. The choices are:
1) Do Not Track - customers can order as much of a particular product as they want and no error will appear if they order more than you have in your inventory.
2) Backorder - will allow customers to order as much as they want but a warning will appear if you run out of stock, but will continue to let customers order as much as they want.
3) Out of Stock - will continue to the show the item when out of stock, but will display "Out of Stock" next to item and will not let customers attempt to purchase the item. Customers can only order up to the amount of stock you have left.
4) Hide Product - will hide your product from your store once the product is sold out. Customers can only order up to the amount of stock you have left.
5) Track Attempts - will allow customers to order as much as they want, no matter the stock. Once a customer orders more than what you have left in stock, it will let them know that they paid a deposit on the product. Also, it will log the attempts to over-purchase your product, which can be viewed at Store->Inventory->Attempts
6) Pre-order - will allow customers to order as much as they want, no matter the stock. Customers are also charged a deposit amount which you can set at the time of setting the product setting.
This issue you have asked about can be prevented by:
1) Going to Store->View Products in your AllProWebTools console
2) Click on the ID number of your product; this will bring you to the Edit Product page for that particular product
3) Go to the Inventory tab and press "Inventory Settings"
4) Change the status field to a setting that better fits your needs (i.e. "Out of Stock" or "Hide Product")