Disabling external mail

There could be several answers to this question.

You could block them from routing any traffic outside the organization with a specific deny on their IP address at the firewall.

You could remove their ability to send or recieve email at the mail server.

Really need to know more about your environment to give you a better answer.

See this article:

http://www.petri.co.il/preventing_exchange_2000_2003_from_relaying.htm

To just prevent individual users from receiving external mail look here:

http://www.petri.co.il/block_incoming_internet_mail_to_specific_users_or_groups.htm

Cheers.

RW