It’s incidental censoring. The content can and will move elsewhere, so it’s not preventing speech just limiting where it can go.

Yes the US has a mechanism to ban a company and that should apply to that company’s app. It’s just trade restrictions like with Huawei. Banning a website will be harder, but by banning advertisers from paying for ads it’ll destroy the profitability of serving American consumers.

Apart from what vineyardmike mentioned, there's a few additional levers, like having the app removed from app stores. I don't know what the current state of their traffic and DNS filtering capabilities, but if they really wanted to go nuclear they have jurisdiction over the com TLD. Wouldn't be the first time they've seized a domain on "national security, etc." grounds.