US companies withdraw from European markets. It's an interesting idea though I think that a lot of companies really don't want to leave money on the table.
The E.U. is a pretty big and wealthy market so its definitely not inconsequential to pull out.
The bigger problem is the vacuum left behind will either be filled by the Chinese or home grown companies that could grow up to be strong competitors. Neither outcome is desirable.
However if the E.U. starts imposing fines larger than the benefit of having a presence in the E.U. then U.S. companies may choose to leave. It is also clear that the U.S. will have no choice but to strongly retaliate against E.U. companies selling in the U.S., a path that quickly spirals out of control globally as trade barriers go up everywhere and nearly everyone gets much poorer.
To paraphrase Archer, "Do you want a great depression? Because this is how you get great depressions!"
They'd just say 'Byeee!' There is nothing the US can do that would not harm it more than it harms the EU. It would be far better for the US and EU to put a bit more effort into understanding how the other works and what is important to them. The EU is in essence socialist rather than capitalist and forgetting that can be extremely costly. That has been hidden by the UK which is closer aligned to the US way of doing things.