[exim] change username for mail only

Is there a simple way to change a username for received mail only?
In other words, a user foo@??? does not want to see any mail
addressed to foo@??? but *does* want to receive mail addressed
to bar@???. Obviously an alias will add the second username,
but how do you dump mail addressed to their real username?
--
Jay Rouman (jsr@??? jsr@??? NIC Handle: JSR)
System Administrator, Education Central/Edzone (jsr@???)