I think it all comes down to FCC’s interpretation of the rules they are making. They could enforce it by charging the companies with breaking FCC rules and regulations. That can and would lead to fines, lawsuits, and potential other escalation methods.
In terms of how to block, it will probably start by the FCC relying on the companies to not offer the firmware updates to US residents. This could be then up for interpretation by the company to enforce FCC’s intent, by making some technical or legal maneuver (i.e. updating terms of service, license/usage agreements on downloading/using the new firmware). Then the enforcement actions become steadily more aggressive if those kind of actions do not meet FCC’s interpretation, such as geo-fencing, and blocking of things like VPNs….