I'll tell you what I have done with at least one org. You have no idea what a junk mail nightmare I've been through with them - even while I was a full-fledged Scientologist. But when I left, I was of course even more irritated. I took the materials they sent and ripped it into little pieces, making sure the piece with my name and address was still in tact and legible. And they sent me a lot of materials. I stuffed it all into their postage paid return envelope, along with a post-it that said, "TAKE ME OFF YOUR FUCKING MAILING LIST!" The envelope was good and heavy too. They have to pay for that and there was some satisfaction in that I suppose.
But of course, that did not work.
I have engaged them dozens and dozens of times on the phone, and in person, both when I was in and since I was out, pleading with them to fix their files or to take me off their lists because I was in several orgs' databases in triplicate, sometimes more! I'd get mail from Flag, AO, CC, LA Org for up to four variations of my name, plus my partner's name, and two other family members who once took a introductory course 15 years before. Why couldn't/can't they get their system straight?? Once when I was in, I brought a huge stack of Celebrity magazines delivered to my house into the Pres' Office at CC demanding they straighten this out, and
nope, nothing. My mailbox would still be so stuffed I could hardly open it at times.
(Keep in mind, someone on staff is counting each of these mailers as a stat and another member of Scientology among the millions upon the earth.)
I looked into filing this thing at the post office, and I would have done this. I was about to. You can claim you are receiving obscene mail. They would have to stop. But this was going on more than a decade after I left the group and then I found myself on the phone with someone from SCN INT. We had a number of conversations over the course of a month and I not only informed him that I was writing a book, but also if he didn't do something about the mail situation, I was going to call my lawyer. It was more involved than that, but
somehow... the mail began to lessen. I haven't seen any in a long time. And I mean probably a year or more now.