How to create categories where you can't post an ad?

I may be looking at this the wrong way, but for the life of me I cannot figure out how to create containers or categories where you cannot enter an ad. For example, I created a category of Chevrolet, and a subcategory of Camaro.

Chevrolet -> Camaro

I would like a user to be able to enter an ad under the Camaro category, but the Chevrolet category should be off limits to post an ad. Is this possible somehow?

Basically I only want someone to post an ad in the deepest nested category.