How to disable internet mail for a group of users on Exchange 2003

Hi,

We have a group of users in a call centre that have been given access to email to communicate with call centre managers only.  They should also not be able to send or receive emails from external recipients at all.  I was able to change the accounts to only allow the call centre managers to send emails to these users, but blocking the emails on the other side of the coin is proving to be a bit more difficult.  That being the emails being sent externally.  We have Windows 2003 Server running Exchange server 2003 SP2.

I found the following and also had a look at the associated write up and links.
http://www.amset.info/exchange/restrict-internetmail.asp

I have created a group call Restricted Email Users and added all of the call centre users to this group.  I have then tried going into the SMTP connector and tried changing the following:

Access -> Authentication -> Users and then added the Restricted Email Users group with deny all.  from here I tried restarting the SMTP connector as well as the message routing service and the users are still able to send the emails to external public addresses.

I am time poor at present and would appreciate a bit more info that might help me get this sorted - what to try next or am I looking in the wrong spot.  

Thanks,