Recently we came across an issue where we had used the Exchange Management Console to remove full mailbox access from a number of users, whilst this completed successfully users advised they still had the mailbox showing in Outlook as an additional mailbox and when they tried to remove it via Account Settings it would simply reappear.
In order to remove this we need to use powershell, the exchange management shell to be precise, first you must give the user back full rights to the mailbox in question but use the extra parameter to remove the auto mapping feature:
Add-MailboxPermission -Identity <mailbox alias of additional mailbox> -User <UserAffected by Extra Mailbox> -AccessRights FullAccess -InheritanceType All -Automapping $false
Next we need to remove the users full access permissions:
Remove-MailboxPermission -Identity <mailbox alias of additional mailbox> -User <UserAffected by Extra Mailbox> -AccessRights FullAccess -InheritanceType All
Now the additional mailbox will be removed, this may require a restart of Outlook.