The mailing list is currently set up as a simple yes/no option for all users. To unsubscribe a user, you need to check an opt-out button on their user account page.
To find a user's account page, go to the User List using this link. (You can also navigate there by going to Administer > User Management > User List)
On that page, enter a user's email address, then click "Apply." Optionally, you could enter their first or last name, or sort by their role.
Once you've found your user in the list, click on the Edit link under the Operations column. That will take you to the user's account profile. There, in the "Email Opt-Out" settings box, check the box to "Opt out of site email". Then click Save at the bottom of the page. The user is now unsubscribed from the NACLA email list.