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.