As an admin occasionally you’ll give yourself permissions to a mailbox. And then you’ll remember Outlook conveniently auto-maps that mailbox for you. Easy fix.
Close Outlook. Open Powershell.
Add-MailboxPermission -Identity targetuser@foo -User youradmin@foo -AccessRights FullAccess -AutoMapping:$false
Open Outlook. Enjoy the emptiness of the side bar.
You can/should also (once the mailbox disappears from Outlook) remove the permissions completely.
Remove-MailboxPermission -Identity targetuser@foo -User youradmin@foo -AccessRights FullAccess -InheritanceType All
