Disable 'None (top category)' from Parent Category in category menu

Hi.

In a category menu set-up it is possible to disable the adding of records, and to limit the number of records that can be added.

I would like my user to be able to add an unlimited number of subcategories, but not be able to add parent categories.

For example. I will create Parent Categories 1,2, and 3 before passing it over to my client. The client would be able to add as many subs as they wish to the existing Parents, but would not be able to add a Parent Category 4.

Parent Category 1
Sub 1a
Sub 1b

Parent Category 2
Sub 2a
Sub 2b

Parent Category 3
Sub 3a
Sub 3b

It's not vital, it would just be handy to have to keep some clients under control who have a tendancy to add loads of pages.

Thanks