Limit categories to hierarchal depth of 2

Is there a way to limit the standard post categories to only allow one level of subcategories?

For example, allow the following:

Fruit 
> Apple

Veg
> Lettuce  

But from here, do not allow "Apple" or "Lettuce" to be a parent.

Is this possible and how can I do it?

Edit: The categories will be the primary method of navigation and I want to stop clients from adding endless levels of sub-categories and overcomplicating the navigation. I know I could just tell them, but I think we all know what clients are like. 1 level of sub-category is useful in this case, 10 is a mess.