stop particular user from receiving outside email

i am using  windows 2000 server as  for active  directory and windows 2003 for excahnge , i have 180  emails users in my  domain, by default all users in AD  can send and receivie emails i mean external  from yahoo gmail  hotmail, but many users are using company faility  for personal reason  so  i would  like some users only to receive  emails and some i don't want them to send external emails  like to yahoo, hotmail.  intenrally they sould be able to send and receive  emails within domain. pleas help where to configure  the settings.