You'd be surprised how many companies or individuals won't exchange money with you if doing so puts them in criminal or civil legal jeopardy. No need for even a Great Firewall.

If US suspects they are breaking the law they can convince judge to sign warrant to get that information or start lawsuit and go through discovery. If they refuse the judge can hold them in contempt of court. I assume next they could just get judgement against them (assuming they are breaking the law) and that could be e.g. require seizing assets and dissolution of the US company.

By banning them from operating in the US. The implementation really isn’t complicated - it’s a simple statute outlawing the company on national security grounds, and all the tech companies (viz Apple and Google) will have to abide by it or face huge fines and criminal sanctions.