Removing the “Additional Mailbox” using the Advanced Outlook settings some times doesn’t unlink extra mailbox from the Outlook profile. To manually remove it, you need to use the ADSI Edit tool.

• Run adsiedit.msc on the server and connect to the
  Default naming context [Server FQDN] (not under Configuration or Schema).
• Browse to the OU where the user account you want to stop sharing is located.
• Right-click “CN=UserName” and select Properties.
• Scroll down to the ‘msExchDelegateListLink’ attribute and edit it.
• Select the user you would like to stop sharing the mailbox with and click Remove.
• Exit by clicking OK, OK and close the ADSI Edit tool.
• Open Outlook on the removed user and wait for a shot period of time. The shared mailbox will be removed.