In your list header, in the Notebook=, keword, ensure it says "private"
which restricts it those subscribed.

This does not solve your problem for those who vacuume, and I don't
know of one which truly will; LISTSERV really can't tell who is legit
and who is subbing to take advantage.

I know this goes on with my lists and I don't like it, but the tightrope
here can be well greased.  List owners know that, though, so being yelled
at from three different directions for the same action is piffle.

Umm, sub by owner, if you care to do the work?

Douglas