By specifying an Org Unit for a person you can use role permissions to limit them to the section of the hierarchy that is their own Org Unit and below.

This can lead to an odd situation though, where a privileged user who has had an Org Unit specified has permissions to manage the entire Org Structure but can see only a small segment of it.

In this situation there is almost certainly no need for them to be restricted to a part of the hierarchy (indeed the opposite is true, they probably need to see the whole thing), and so if this arises then the fix is simply to remove the Org Unit entry from their Staff record.