That's easy. If spacex does not respect the ban, China will fire anti-satellite missiles to the spacex constellation.
More feasible attacks would be hacking the satellites or ground systems, RF jamming, or just locating users on the ground by their RF emissions and throwing them in the gulag. Or how about holding the Shanghai Tesla factory hostage? Less fancifully, China can put pressure on SpaceX via the ITU. There are plenty of options for China and there is zero chance SpaceX will try to break Chinese law.
Every country, US included, would pressure SpaceX to stop operating in countries that don't want it if it were to result in any debris.
no no
no no no
please. if you want to make Starlink illegal what you do is state Starlink is illegal and maybe make it illegal to pay Starlink. that's a lot cheaper and doesn't piss off the international community.