Step 2: Hide the Filesystem Media Source
Now it's time to hide everything above the "/assets/files/" folder.
Go to Tools > Media Sources.
Right click on the Filesystem Media Source and select "Update Media Source".
Go to the Access Permissions tab.
Click "Add User Group"
Select "Administrator" as the user group, "Member" as the minimum role, and "Media Source Admin" as the policy. This will mean that all users in the Administrator user group will be able to add / remove media sources. You want to ensure that your clients are in a different user group. If they are in the admin user group, you can set the minimum role for this media source to "Super User" so that only users with a role of "Super User" can access the media source.
Click "Done", and then "Save".
This question has been answered by lucy. See the first response.