Office 365 Shared Calendar Read Only

I created a Shared Mailbox for the purpose of using it as a Shared Calendar. I need to make the Calendar Read Only to certain individuals. In powershell I enter the following command: Add-MailboxFolderPermission –Identity <User:\calendar> -AccessRight ReadItems -User <User\Group Name>

After I set the permissions I proceed to add the Shared Mailbox to Outlook for, but they are still able to make changes to the calendar. What am I doing wrong? I need to the calendar to be Read Only

We are using Outlook 2010 as the client.