Not sure about Vista but in XP its a registry setting I think. I loaded up RegEdit and searched for my MSN username. I got a result in HKEY_CURRENT_USER\Software\Microsoft\IdentityCRL\UserExtendedProperties\.
So if you can get at the Vista registry and search for your MSN usernames that you want rid off, you should be able to delete the keys. Back up the registry first though, if you can.