The method that I'm using this year:

* SUBSCRIPTION= CLOSED
* SEND= OWNERS

Sent a note to [log in to unmask] with an explanation (folks can still
unsubscribe themselves).

Also updated the listname.MAILTPL section

>>> SUB_CLOSED Message sent to user trying SUBSCRIBE on closed list
Sorry, the &LISTNAME list is closed. Contact the list owner
(&OWNER) for more information, BUT before you do, please read the
following:

We're taking &LISTNAME on holiday so we don't have to deal with
the multitude of bounces that invariably occur when a
disproportionate number of subscribers have email problems.

Thank you for your understanding and please retry 1/5/1999.