sending mail with a different name

In the office we’re using exchange 2k and outlook 2k/XP. In advanced mail option you can add people who can read and send mail using your name. But, when you go to view – field From and select this one, you can send a mail messege from any user in the global list, external contacts included. How to disable this field From option, i really can’t find it. (i hope the translaten from dutch to english is correct) Thanks in advance!