I use Windows 7, Outlook 2007 and to remove those unwanted addresses I do this:
Click on To, mouse down to the address you want to remove in the drop down box,
Left click and hold the mouse button (the address will be overlaid in blue),
While holding the left mouse button down, hit the delete key.
The address will now be deleted and one of the addresses in the drop down box will end up in the To area, just delete it from the message To box, unless you want to email them something.
The biggest thing to remember to do is not to release the left mouse button; you must keep the address to be deleted highlighted.
Good luck, and let me know if this works for you. If not, oh well.