USPS says you can write “Refused” on your mail to refuse it. I’ve heard that can possibly stop them from sending you more.
If you wanted to actually opt-out from the largest mail advertising organization’s lists though, you have to pay them. Good 'ol capitalism.
The site is OptOutPrescreen, by the way!
It’s not a federal website, it’s a “joint venture among Equifax Information Services, LLC, Experian Information Solutions, Inc., Innovis Data Solutions, Inc., and TransUnion, LLC (collectively the “Consumer Credit Reporting Companies”).”
So it’s not technically government-enforced in any way, but the credit bureaus just kind of decide to honor it to be… nice?
I know nothing about this but top of my head it sounds like one of those self-policing things companies do to keep from being regulated. So they’ll honor it if you jump their hoops, but so few people even know the hoops are there it works out for said company to honor the few that do, and they will, to keep that regulation from ever appearing.