I'm by no means an expert, but from my limited understanding there isn't so much an EU "authority" that prevents this, but rather a large group of rules that ea

I'm by no means an expert, but from my limited understanding there isn't so much an EU "authority" that prevents this, but rather a large group of rules that each country agreed to. In particular, the agreement that protects greece from protectionism is really just an agreement by the member countries to set things like tariffs exclusively by mutual agreement. In other words: it's all or nothing (within the vagaries all kinds of legalese); so no; greece cannot go it alone without exiting the union.