The issue is not that they can't be stopped, but that it is not in Germanys interest to stop them. They need the energy, just not in the North where it is produced. They just don't have the North-South transmission capacity, and so the energy flows through neighbouring countries and put their neighbours networks at risk.

As long as their neighbours let them, it's vastly cheaper for Germany to keep this going than to speed up the upgrades of their own transmission capacity.