[exim] Aliases and local delivery

Good day,

I have machine which is receiving mail for two domains. Mail accounts
are mapped to unix users. I would like to achieve with help
of /etc/aliases to specify only one valid email per mailbox.

address user@??? is being mapped in /etc/aliases as
user@???: user

address user@??? isn't mapped in /etc/aliases and is anyway
delivered to user's mailbox. (the same one)

how to tell exim not to deliver user@??? at all?
I'm using exim 3.

here is relevant part of exim.conf:

system_aliases:
domains = @
driver = aliasfile
file = /etc/aliases
search_type = lsearch



--
Zbynek Houska <zbynh@???>