Office 365 Resource Mailbox - Default User - Default Permissions

Greetings.  We have multiple resource mailboxes set up for booking conference rooms.  As resources, they have the auto-attendant feature and check free/busy information.

I just found out that many end users have been booking their meetings (and changing/deleting) them directly in those resource mailbox calendars.  This is referred to as "Direct Booking" if I'm not mistaken.

I'm pretty sure this is a no-no and something I *do not* want end users to do.  I want them to schedule their meetings in their own calendars, then invite the resource (room) so that the auto-attendant will tell them if the meeting is "accepted" or "declined" in that room.

Here is my question. The rooms were set up with permissions for the "Default" all over the place - Editor, Custom, Contributor, etc.  I've read that I should change the permissions to "None" for Default.  I presume this still allows the room to be invited as a resource and have the system attendant work, but if they view the room directly, it won't show free/busy with the "None" permission.

P.S. We're using Outlook 2013 as the client.

Thoughts ? Suggestions ?
Thanks much.
-Stephen