Limiting a Listing Package to a Specific Category

I have an attorney directory with the general, parent category of “Lawyers” and then many sub-category practice areas. I would like to allow members to list in the parent category “Lawyers” for free. However, the only limitation I see is to limit them to one category which doesn’t preclude them from choosing a single sub-category instead of the parent category “Lawyers.”

Is there a way to allow users to list in the parent category for free without allowing them to choose a sub-category instead?

Thank you for your time.