How to disable Unix users?

[Marek Kozlowski]

:-)
Maybe a stupid question...
I'd like to allow incoming (or local) mail only addressed to aliases
specified in `vitual_alias_maps' and `alias_map' but reject recipient
addresses using user names.
For example I have a user `smithj' which has an alias (or virtual)
`john.smith'(@...). I'd like to accept mail to `john.smith@...' but
reject mail to `smithj@...'. Maybe I'm blind but I can't find the option
/ way how to do it...
Best regrads,
Marek