Hi
If you use a stock management, the quantity ' availability ' is the physical inventory of the product, i am that it is possible to order more, the assumption that the relicat would be reserved.
I have not provided the case to disable the add to cart if more than physical inventory, but this is not very complicated to intercept the addtocart event.
As soon as i have time rework on this code i would add this option, i probably move the project on Google Code, more easy with SVN repository