It's fairly easy these days to get them to stop, reading OTIII may not work though because most of the people ringing you would have no idea what you were talking about because they haven't done OTIII themselves and Uncle Xenu (try mentioning him, he is OTIII related) is supposedly still a huge secret (lol, they would probably think you were a nutter though and remove you from their lists anyway, so it could be worth a try!).
If you want to make it simple and fast just say something along the lines of "please remove me from all of your lists immediately, I am not and never will be a scientologist because you are a greedy cult full of idiots and I will go to the Police if I hear from you ever again" ... and hang up.
That should do it.
But, I think telling them some simple sentences using Xenu and body-thetans in the sentences will cause them some long range pondering and stick in their heads, to their "supposed" detriment, and instead might lead to the breaking down of Scientology's "information control" over their members.
I favor these spoken or written sentences, which even the lowest level Scientologists will understand, it might stick in their heads, all their Scientology careers:
a) Xenu caused the Wall of Fire
b) Xenu caused earth's body-thetans problem, for which Scientology's OT 3, 4, 5, 6, and 7 "upper levels" are exorcism levels that the Scientologists do on themselves at their "Advanced Orgs" and at "Flag" to rid themselves of Xenu's body-thetans they have infesting them.
c) Stop contacting me, I don't believe Xenu caused the Wall of Fire, nor did Xenu dump body-thetans on earth, nor that Xenu's body-thetans infest humans, nor that humans need to do Scientology's OT levels 3, 4, 5, 6 and 7 exorcism "advanced" levels to rid themselves of these non-existent Xenu body-thetans.
If the average outside non Scientologist world gets the word, that Scientology's "advanced levels" are exorcism of Xenu's body-thetans, and that the world knows the big secrets of Scientology, and if they public can simply relay the simple truth of Xenu's relationship to the supposed earth dumping of body-thetans that infest all humans today, and and that the secret exorcism levels consist of OT 3, 4, 5, 6 and 7, this information is understandable to the lower level Scientologists.
And it severely gives them their needed spoiler alert about their secret religion that isn't secret.
But the public, widely, has to get the Xenu's body-thetans spoiler alert, so that this percolates back into Scientology and breaks down their totalitarian information control over the "secret" (no longer) Xenu and body-thetans theory.