I actually received a letter back from the Postmaster today, pretty much confirming what I thought. They said that they have to deliver all mail with a correct address and postage, and suggested I write the people sending me mail to tell them to stop mailing me. I’ve done this quite a bit, but it’s a pain to mail everybody, and it doesn’t stop saturation mailings (where a place pays the post office to just stick a copy in everybody’s box, and the address is just “Postal Customer” so they can’t just not print an address label for me). They also reminded me that I can refuse mail being sent to me by marking it Refused and putting it back (which is what I’ve been doing all along, and no doubt the Postmaster knows it).
So, I guess I’ll continue trying to opt out of mailing lists and refusing all the mail I don’t want.
I do wonder how much of a business opportunity there is for a private mailbox place (a private Post Office box-like rental, which does exist) which does “spam filtering” for its customers based on rules, and ideally complicated OCRing of return addresses rather than trusting humans to apply the rules correctly (and probably online access to the scanned rejected images for some period of time allowing for retrieval of false positives).