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.