They can certainly quit giving US astronauts rides on their rockets, but the International Space Station is like 75% US made, so I’m not sure where they have the ability to ban the US from using it once Elon Musk’s Dragon capsules are fully operational. FWIW, NASA has the ability to drop the ISS into the Pacific Ocean any time they want to.
“NASA has the ability to drop the ISS into the Pacific Ocean any time they want to.”
I doubt that very much...