Is it any of your business, as list owner, what people send off-list?
Even if it's in response to list traffic? Who appointed you the
censor of all that someone else sends?
Lot's of very interesting opinions here, across the spectrum
and all have merit.
The end-to-end principle would seem to indicate that mail sent
to your list should pass through unhindered, and if the recipient
chooses to reply to the sender, that's between the recipient and
Of course, if you (as the list manager) are acting as a moderator,
editor, or other active role (I guess so, if you're asking here before
taking action) then not only should you be allowed to censor content to
further the aims of the list, your subscribers
probably expect you to do so.
Would it be possible (and appropriate) to switch out-of-office
subscribers to "digest" mode? That should prevent _anyone_ from
receiving an out-of-office reply (except perhaps you, or the list if
reply-to is set such). Plus, you haven't censored or uns+bscr+bed
anyone. It still leaves you with the problem of finding-out who
is auto-responding, but once you know , this may give you a least-impact
way of eliminating the problem.
 Not that there's much end-to-end left of the Internet today...