I'm not sure if this was the answer you were looking for but I’ll take a shot
anyways. When you type in a name or address into the To: field and you want
to get rid of some erroneous names, just make sure the name is highlighted
and hit the Delete key on your keyboard. If your question was to simply
disable the AutoComplete on the To: field, what you have to do is the
following (in Outlook 2003). Go to Tools --> Options. In the Preference tab
select E-mail Options. From there click on Advanced e-mail options. You'll
see near the bottom some options relating to Automatic name checking and
Suggest names while completing to. Deselect the desired option and you should
not get the AutoComplete feature anymore.