This isn't a societal-level solution, but if you want to greatly reduce the amount of junk mail you get I can personally recommend this guide:

https://ecocycle.org/eco-living/refuse-and-reduce/stop-junk-mail/

It goes into detail about some individual junk mail companies later in the guide (Valpak was the one I needed) and returning to sender and whatnot, but just doing the DMAchoice opt-out and the credit/insurance offer opt-out nearly eliminated the junk mail I get. All the other stuff is for cleaning up the few stragglers that remain.