[numakerg]

My naive suggestion. "Hard" unsubscribe button that tosses all future emails from this list in the trash and mails the sender a note that the address has unsubscribed from this list and all future emails will be ignored.

Gmail has over a billion active users. Mailing lists will probably adapt to whatever crumbs Google leaves on the doorstep.

[zbentley]

Isn't that exactly the "mark as spam" button?

[numakerg]

Mark as spam doesn't notify the sender