USPS form 1500 https://about.usps.com/forms/ps1500.pdf can be useful in combatting junk mail as well. It's technically for pornographic material sent through the mail, but the definition of "pornographic" is wholly upon the person submitting the form. The only time I ever had to use it was with AT&T - they simply would not stop sending mail, every day, sometimes 2-3 a day. I emailed their customer service, was told they can't do anything, and I simply said "That's fine, I'm going to file a 1500 with every letter I get." I don't know if it was the threat or my email simply got to someone who could do it, but I received a reply a few days later saying my mailing address had been removed, and to wait at least 90 days for mail to clear the system. A clear business letter using the Inigo Montoya system works well for most things.