To turn off this feature, so that addresses are never suggested, go to the Tools pull-down menu, choose Options, then click the E-Mail Options button, and the Advanced E-mail Options button. Uncheck the box for "Suggest names while completing the To, Cc and Bcc fields".
If you want to remove one address from the displayed list so that it won't be suggested again, open a new message and begin typing in the address. Use the up and down arrow keys to select the address to be removed from the addresses suggested. When the address to be removed is highlighted, hit the Delete key.
To remove all the saved addresses, delete the .nk2 file where Outlook stores them. The file is normally located on c:\Documents and Settings\username\Application Data\Microsoft\Outlook