[xwiki-users] How to avoid nested space?

How to avoid nested space? Since the new nested space feature, a lot of nested space such as "XXX.YYY" appears in the "Space List" in my home page. If I want to create a sub page, a new nested space created silently. It becomes more and more, and bothers our users. I want keep the original simple one level space and documents tree model instead of the new nested space model. How can I get it?