$allRooms = Get-Mailbox -RecipientTypeDetails RoomMailbox

$allRooms | Format-Table

$allRooms | ForEach {Set-MailboxFolderPermission -Identity "$($_.alias):Calendar" -User default -AccessRights "LimitedDetails"}

$allRooms | Set-CalendarProcessing -DeleteSubject $false -DeleteComments $false