Can't do it. The "Main Category" and "Sub Category" are covered by the same "class". Anything I want to do to the Sub Category menu item will affect the Main Category item too.
e.g
.left-sidebar ul a {
display:none;
}
This will make BOTH the Main Category items and the Sub category items disappear. The Sub Category menu item will have to have a separate class assigned to it by modifying the core theme files. Too much hassle.
Also ... it just occurred to me. If the customer can't see the Sub Category in the menu, they won't be able to navigate to that Sub Category. If they can't do that, they won't see the products in that sub category. If they can't see the products ... they won't buy them.
It's really not a good idea in my opinion.
So I give up. Spent too much time on a bad idea already ... LOL