Categories and subcategories

Hi,
On MJ theme I’ve created Main Categories and subcategories.
Is there anyway to forbid main category selection when creating a new listing and allow just subcategories selection, but displaying the total number of jobs available in all subcategories?

for example I have:

Repairs (main category) displaying 0 jobs
Phone cell repair (subcategory) displaying 1 job

I wish to forbid, when an user creates a new list, the possibility to select the main category, while allowing to select subcategory for the new list.
Also would like the total job count for all subcategories to be displayed on main category.

Which part of the code takes care of this?
Can anyone of the gurus suggest a solution?