This is a social problem which cannot be resolved solely by technology.

We have attempted to address this issue for a department with an internal
announcement list by setting up a weekly job to refresh the list, i.e.,
delete all subscribers, then bulk-add everyone who should be on the list.

The list owners know we cannot prevent subscribers from leaving the list
or setting their subscriptions to NOMAIL, but this is a low-volume list,
so the weekly refresh provides a relatively high level of assurance that
messages posted to the list will be sent to everyone who should see them.
The list owners take the position that they are making a good faith attempt
to ensure that messages are delivered. If some recipients choose to filter,
delete, or ignore messages, those recipients will have to accept any
adverse consequences.

The same problem exists with paper mail - it may be possible to distribute
an announcement to everyone who should see it, but it is virtually
impossible to guarantee that everyone will read it, comprehend it, and
respond in the desired manner.

--
Paul Russell
Senior Systems Administrator
University of Notre Dame
http://www.nd.edu/~prussell