Block users from Internet mail

I run an 2000 Small Biz Server with Exchange 2000
All of the users in the office need internal E-Mail. So I set up those internal users with a
domain.local primary address and the users that need internet mail with a domain.com primary address. Well the users that are on the domain.local address's can still send Internet mail although they can not recieve any internet mail.

How do I Block them from Sending Internet Mail?
They are already Restricted from sending messeges on the SMTP Connector on Exchange, But it doesn't work. Help. Thanks