[Exim] Allow alias@domain, disable username@domain

Hi,

I'm trying to reduce the number of valid adresses for several domains hosted on
one server. Firstname.Lastname@domain should work, username@domain should not
work for people sending mail to it (but exim should be able to deliver the
expanded aliases).

The host has about 1000 unix users and 5 domains, that is 5000 valid adresses +
the official adresses (aliases). Spam is a problem...

Exim 3.3.x is in production, but will upgrade to 4.x soon (or now, if someone
know how to do this in 4.0).

I'm not sure if I'm on the right track.. The "no_more" in the following wil stop
all delivery, but I feel that I might be close..??

virtual:
domains = "lsearch;/etc/exim/aliases/domains"
driver = aliasfile
file = /etc/exim/aliases/$domain-aliases
search_type = lsearch
no_more

I'we searched the FAQ and arcives, but I can't find what I'm looking for.

Regards
Kristian