Do you mean, who's stopping them from not collecting personal data? No one, that's the point. If you're not collecting personal data none of this applies and you can serve whatever you want to people in the EU.

If you mean, who is stopping them from handing over data to the US government? That's exactly what this court case is about. They can't conduct commerce in the EU unless they have a mechanism to avoid that, and progressively more strict enforcement gets imposed by courts if they keep trying. (Eventually, presumably being detained if they try to enter an EU country, though I seriously doubt it would escalate that far in practice.)