How to set a restriction for selling more products than are available in a warehouse

In order to set a restriction for selling more products than are available in a warehouse go to Warehouse > Products > Edit. 

Now go to the bottom of the screen, click See more options and check Quantitative restrictions (warehouse).

Turning on this feature will restrict the warehouse from selling more products than there are available.

This feature is useful when you have a limited amount products to sell or if you want to restrict a product to a specific warehouse.