RM 1.2 How to exclude (sub category) resources?

I asked about this before but this time I would like to request if anybody able or know how to do this.

In RM I have:
- Category I
- - Sub Category I

If I enter (Category I) then I will see all the items inside (Category I) & (Sub Category I)
What I want is: if I enter (Category I) I want to see the items inside (Category I) only without (Sub Category I).

We have ~400 categories, 6000 files and it is something critical for us to exclude the sub category from the main category resources.