We will have to assume your Outlook version is 2002 or higher.
Remove the incorrect entries from the autocompletion cache.
Remove individual addresses from the autocompletion cache by highlighting
the entry when presented in the suggested names list (use your arrow key to
migrate to the entry) and then hitting your Delete key before you tab out of
the field. To clear the entire cache, follow the instructions here:
http://support.microsoft.com/default.aspx?scid=kb;EN-US;Q287623