If Google don't pay then I guess the EU could try to sieze the bits of Google that are in Europe and sell their assets to pay the fine. Or start arresting their executives whenever they fly through European airspace. Or stop EU companies from paying them.
If Google want to operate in the EU, and they do because they make profits there, then they'll have to pay up.
Also Google get away with being massive privacy thieves and data-hoarders through inertia. They're useful, and it's a lot of hassle to regulat them. But if they tried to pull a stunt like that, the gloves would come off pretty quickly.