The smtp service on any linux/unix system is served by a MTA (Mail Transit Agent). In the case of a system running DA, that MTA is exim.
It runs server wide.
It cannot be shut off for a specific user.
However, depending on what you want that user to not be able to do, you may be able to do it.
Exactly what do you want the user to not be able to do?
Send email, receive email, what?
Jeff