You absolutely can, because the companies providing tech services need to get paid, either from local companies buying ad space, or directly from their users, and you can just block payment to non-compliant firms.

Given America's hostile stance to its former allies, joint ownership and technology transfer agreements as a condition for its firms operating in the EU market sounds more than reasonable. If US firms aren't happy with this, they can always lobby to impose similar restrictions on foreign big tech firms operating in the US...

If there were any.

The problem with flipping the table and starting a trade war is everyone loses... And the correct response to someone starting lose-lose games is tit-for-tat. Maybe big tech firms will think twice before throwing their support for an anti-trade candidate. (They absolutely could have used their position in media to stop this insanity, if they wanted to, but now we all have to lie in the bed they've made.)