[Exim] disabling a system user

Howdy group,

I know this is simple but I just can't seem to find it.

What do I add to the configuration file if I want to make it so a system
user (IE exists in /etc/passwd) can not send mail at all period whether
its through a script or a direct SMTP session even if it authenticated
via SMTP.

IE if I gave you the user, pass and hostname you could not use that info
to send mail.

TIA

Lee.M