Stopping the ability to create a new sub-wiki page.

In a Wiki category, I want users to be able to create new pages...that one is straight forward. How do I stop them from creating sub-pages?

For instance:
Category = Trees (Users can create pages in this category)

Oak - This is a page under "Trees". I don't want people to be able to create a page under "Oak". They can edit the page, but not create a sub-page.