Limit users from send/rec internet email

We have a full blown Exchange email system and Outlook 2000 clients. We want to be able to limit some users from sending or receiving email through the internet (only local email will be allowed for these users). Most of these users are temps and wedo not want then filtering out company information to the internet. They should be able to send email to other users internally. What is the best way to implement a system like this?