To disable internal email, you can hide the address from the global address list and but the external address will still be valid and the mailbox will receive it.
Moto, hiding the address from the GAL won’t prevent those who have previously emailed that person from doing so again. I don’t know of a way to do this is Exchange 2003. It’s easy to do in Exchange 2007.
I would remove their cached email addresses or at least delete that one for the user from their cached addresses. other than that, i’m not sure how it could be done.