Limit permission on calendar

Hello,

I create  ressources (equipments) mailboxes on my server. And I want to restrict the reservation of this equipment. I mean one or two groups (AD security groups)  should had the permission to book this equipment.

I use this commande: 

ADD-MailboxFolderPermission -Identity My-equipment:\calendar -User 'security_group' -AccessRights 'owner'

Set-MailboxFolderPermission -Identity My-equipment:\calendar -User 'Default' -AccessRights 'None' or 'Reviwer'

When I verify the permission:  Get-MailboxFolderPermission -Identity My-equipment:\calendar

FolderName           User                 AccessRights
----------           ----                 ------------
Calendar             Par défaut           {None}
Calendar             Anonyme              {None}
Calendar             AD_User          {Reviewer}
Calendar             My_security_groupe     {Owner}

BUT, Everyone can book this equipment !! Is there any mistake ? How can I give the booking rights juste for one Ad Security group ??

Thanks