Would it be cheaper for those companies to walk away from european market?
Europeans use America-only firms all the time and can easily make cross border payments. There aren't really robust ways to stop US firms selling to the EU over the internet especially for services, which are excluded from tariffs via GATT.
Whilst I don't think we'll see established firms "walk away" in the sense of closing down their EU offices, I think it's extremely likely that the next generation of startups will simply never choose to open offices in the EU to begin with. Sort of like how many companies ignore the Chinese market today because the problems that come with it offset the appeal of a large (much, much more captive) market.
It’s rather easy to seize transfers made to foreign companies in violation of a local law. So while that foreign company then wouldn’t have to have a presence in the EU, it also couldn’t receive payments from EU companies or customers.
Much the same way the US will complain if you transfer funds out of the US to terrorists or drug lords or whoever is politically unfavourable today.
Yes, if it is willing to go down that route it can seal off people from America entirely. But the fact that people are even seriously proposing this is the best argument for Brexit yet.
I'd say not.
It would hopefully be cheaper to follow the law. If not I'll vote for higher fines :)