The USPS doesn't even have to get involved. Just make it so that recipients can be compensated easily enough with high enough fines and the spam stops immediately.

The USPS literally has a paid service to deliver bulk mail to every address in an area.

And if the law was working correctly that would mean free money for the whole area and a bankrupt sender.

You seem to be suggesting that you're going to successfully fine a postal spam sender for using a paid service offered by the USPS to do the thing that service was designed to do. If the "law was working correctly", that service would stop existing, rather than existing and then incurring fines when used.

There are some limited mechanisms for opting out of targeted mail (e.g. things that have your name on them), and mechanisms for opting out of credit offers in particular. There are not any mechanisms for opting out of "current resident" spam sent to everyone in an area.