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. If your autocomplete list contains only a single entry, use the
directions here:
http://support.microsoft.com/default.aspx?scid=kb;en-us;289975

To clear the entire cache, follow the instructions here:
http://support.microsoft.com/default.aspx?scid=kb;EN-US;Q287623