Exchange's AutoMapping feature allows shared mailboxes to be added automatically to a user's Outlook if that user obtains full access permissions to that mailbox. This eliminates the need for the user to add the mailbox manually in Outlook. Subsequently, the same mechanism is also used when access to a shared mailbox is removed for a particular user. The mailbox should then automatically disappear from Outlook. Unfortunately, this does not always work properly.
Because of an intermittent bug, sometimes it can occur that a shared mailbox is not automatically removed from Outlook after access to that mailbox has been revoked. In fact, manually removing the mailbox (through Registry) doesn't help, and the mailbox gets placed back in Outlook after a few minutes.
The reason for this is that a certain attribute of the shared mailbox still contains a reference to the user's account, causing Exchange to place the mailbox back, even though the user has been denied access to the shared mailbox.
You can manually remove the reference to the user account in the attribute of the account of the shared mailbox in Active Directory, using ADSI Edit.