Disable Double Booking of Meeting Rooms
Set-CalendarProcessing -Identity ‘room’ -AllowConflicts:$false

Disable Double Booking on all Meeting Rooms
Get-Mailbox | where {$_.ResourceType -eq “Room” } | Set-CalendarProcessing -AllowConflicts:$false

Get Status of Meeting Rooms Calendars
Get-CalendarProcessing -Identity room | fl

Get Calendar Permissions for user
Get-MailboxFolderPermission roomname@domain.org:\calendar

Grant read rights to the calender in the “MeetingRoom” calendar
Add-MailboxFolderPermission MeetingRoom@domain.local:\calendar -User Default -AccessRights reviewer

Grant members of the AD group Admins read and write access to the calendar.
(The AD group must be mail enabled and cannot be hidden from the address book when executing the command).
Add-MailboxFolderPermission MeetingRoom@domain.local:\calendar -User Admins -AccessRights editor

Remove access rights for the default user on mailbox
Remove-MailboxFolderPermission MeetingRoom@domain.local:\calendar -User Default -AccessRights editor

Set Reviewer permissions to the default account
Set-MailboxFolderPermission -Identity MeetingRoom@domain.local:\Calendar -User Default -AccessRights Reviewer