Anonymous wrote: "This is what is particularly troubling, that they would take the time to find me despite my not requesting anything."
I can't say exactly how things are done at the Celebrity Centre LA, but I do know that back in the days I worked at a Scientology church in Sweden, staff would go through any mail that was returned with "no longer at this address" and call the registry office to get hold of the new address. If they just let addresses expire when people move, their data base of addresses would shrink, and since they are nuts about keeping statistics for everything, and those statistics must not go down or someone is in trouble, they do not want their number of addresses on the mailing list to be reduced. Thus, they could really go to the trouble of actively looking up your new address, even if you haven't been interested in their services for years.
I think EJ (who is an active Scientologist) is the only one here who says all you need to do is write a nice note saying "no thanks". A number of us seem to have other experiences, that it may at worst require a threat to sue before they act.
If you're sure you don't want anything more to do with them, a possible alternative could be to print out a copy of the Xenu leaflet
and mail it back to them, that ought to put you in the "Dead File" where they relocate names of not nice people who should not get any more mail (they should not do anything else to you, they are way too busy chasing their active enemies)
Or you can do like AgentX9, and just let the mail keep coming for its curiosity value.