<https://faq.usps.com/s/article/Refuse-unwanted-mail-and-remo...>
From the old Junkbusters site, there is a Form 1500 which can be filed against any sender:
<https://web.archive.org/web/19970713104642/http://www.junkbu...>
Form 1500, "Application for Listing and/or Prohibitory Order":
<https://about.usps.com/forms/ps1500.pdf>
Though specified as against "sexually-oriented" material, that has been deemed by courts to be at the sole judgement of the mail recipient.
You can also directly contact bulk senders through the DMA mail preference service, Valpack, RedPlum, and others. See one listing of these here:
<https://www.huffpost.com/entry/how-to-stop-junk-mail_n_5b27b...>
The Form 1500 is the take-off-and-nuke-it-from-orbit option, however.