Controlling use of categories in Classified

I’m testing Classified using Mamp before going live. If I have multi-layered hierarchies of classified, how to I ensure that advertsiers drill down to the correct level?

EG, Cars classified would go Cars> Porsche > 911 etc.

You wouldn’t want advertisers clicking on ‘Cars’ at the topmost level. I’m trying to think ahead to when I create a custom search field for searching ads. and trying to figure wetheer sub-categories or Tags will be a better way