Allowlisting in Ukraine would absolutely work. The Ukrainian government can track all dishes in Ukraine (public and private), and denylist any which are captured. Starlink can come to them with questions about borderline units. Heck, even banning Middle Eastern Starlinks which are being used outside their registered country would be a good start.
He's running a telecom provider. Nobody has even declared actual war on anybody. During the Cold War we had phone lines to Russia, China, you name it. Of course all communications were heavily monitored, like today. If a war was declared by Congress things would be different of course. But they haven't done that in ages. I think WWII was the last time? Pesky legal issues crop up of course. It's better not to declade war apparently.
This is just Elon being a little shit.