How are your "local" users sending email? If they are using SMTP on port 25 then there's no way I can think of to stop a user sending email to a mailbox on the server without also stopping external email from being delivered.
You could prevent this by switching to an external mailserver, I suppose, and the blocking port 25 from all but the external mailserver's IP.
Maybe if you could let us know why you want to do this -- maybe there's some other way around your problem?