Remove other users mailbox from my outlook

As an IT support, sometimes I have to add other user's mailbox to my Outlook when users are not at office .

We are running Exchange 2010, Outlook 2010, Outlook2007 and Outlook 2003.

But since I  add the user, I cannot remove the user's mailbox from my Outlook. When I close the user's mailbox, the error message pops up" this group of folders is associated with an email account. To remove the account, on the Tools menu, click Account setting, select the account, and then click Remove"

 I went to Tools\Acccounting and remove this account, Unfortunately the user's mailbox does not disappear in my outlook. I add and reomve many times, but the user's mailbox are still there. 

 The others mailboxes appear even when I open the outlook in a brand new computer, even I upgrade Outlook 2003 to Outlook 2007 and Outlook 2010.

Any help will be greatly appreciated.