[Exim] vitual domains, specific alias to dev nul?

I am running many virtual domains on exim 3.x
I am using a specific alias file for each virtual domain .. this works fine.
I am a fairly new exim user with limited experience.

A typical setup is
webmaster: aaccount1@localhost
info: aaccount2@localhost
*: aothers@localhost

In the first situation the * is needed to catch legitimate 'unknown' aliases
but I want to drop all incoming mail for specific named aliases.
Is there a command line such as
olduser: dev nul
or any other per-domain way of deleting mail to specific email aliases


Secondly:
I have another domian on the same exim server where all email to unknown
aliases should be quietly deleted.
i.e.
webmaster: accountx1@localhost
info: accounxt2@localhost

Can somebody please point me in the direction of a solution which solves
BOTH of these situations.

Brian