I'm with Deozaan 100% on this.
I've had to ask one friend several times to (at least) use BCC & finally ended up having to be embarrass/shock her in order to get my point across, stating immediately upon receipt of her junk mails that I couldn't believe
didn't know how to use BCC for sending junk mail. I offered to teach her and was pretty pushy about it...she stopped after a couple - OK, a few - of those conversations.
Then there's the, "I'm absolutely, never, ever responding to this garbage" method that I used with another friend.
This means that when they ask you about it, Say you just don't acknowledge mail of that kind...it goes straight to the trash, as there's just not enough time in the day to read it all. But here, you must stick to your guns. If they think you've read one FWD:, you're finished.
Or, play innocent: "Oh, come on, nobody really reads
those things...do they?"
They may not listen very well, or utilize BCC, but our spamming friends do update their mailing lists from time to time, & zero response does say something, sometimes. Good luck!!