How to restrict drill up at report level ?

Hi

I have a report with  a four level heirarchy and have users in all the heirarchy levels.

let the heirarchy be like the following

Network

Zone

Region

Branch

(Network being the highest)

What I need is that, when the network user logs in he wants the report at all levels which can be achieved

But  for a zone level user he must not see the network level data .

Likewise region level user must not see zone level and network level data

How to achieve this at the report level . I tried to change the scope of analysis but still able to drill up to network

Please help

Thanks

Nithin