Specifically, they could stop using the USD a clearing currency... causing hyperinflation in the US.

They could embargo US firms, Visa and Mastercard first. Tech firms second.

They could sell Dutch lithography to China. In fact they could ally with China in return for turning a blind eye on Taiwan.

They could stop spending billions on US arms, in fact that is well underway.

US shot its bolt in Iran.... everyone is much less scared of the US since it attempted to use its force. It can't sustain a single carrier in the gulf with allied countries at its back. It has already spent 80% of its patriot missiles.