Listserv by default will always permit a subscriber to remove him- or
herself from any list. You can, however, remind these subscribers that
it is important to remain on the list by creating a welcome message that
mentions it into the set of list templates.

Listserv will let a list owner know about subscriptions and sign-offs,
so you will be able to track who leaves.

There are other ways, too, but those should be mentioned on the Site
Owners list, lstsrv-l.

andrew