[Exim] blocking a user temporarily

I use exim 3.02 and use the passwd file to validate incoming mail.

Is there a way to _temporarily_ reject a given user name without
removing it from my passwd file?

I have a few user accounts which go inactive temporarily and a few others
which get suspended temporarily and I'd like to return their email
with the "unknown local part" message.
--
Jim Pazarena    mailto:paz@qcislands.net
                http://www.qcislands.net/paz