Prevent selecting a parentless category

Is there a way to prevent a poster from selecting a category which doesn’t have a parent.

For example, if I have the following category structure:
Cat A
– Child A
– Child B
Cat B
Cat C
– Child C
– Child D

I’d only want to let them to select Child A, Child B, Child C, or Child D, not Cat A, Cat B, Cat C. I can do this when I want to save, but I was wondering if there was a way to change the output of the default category widget to reflect this restriction.

Thanks.