> I'm not even sure POTUS can ban a site just by an executive order... unless I'm REALLY weak on US law...

Being ignorant of the special area of law that is “Presidential emergency powers regarding trade with foreign nations” isn't necessarily being really weak on US law generally, though that narrow area of law ends up contravening a lot of what you might correctly understand elsewhere when it is triggered.

It's not clear exactly what the content of the order will be, without which we can't really begin to assess whether he has the power to issue it, but there are certainly, at a minimum, things he could due to obstruct their ability to fully interact with US markets under Presidential emergency powers. Given that Trump's I formal descriptions preceding executive actions have not been a particularly good guide to the details of the actions, I don't know that any deeper analysis wouldn't largely wasted.