Of course, they may declare the Bank of Greece insolvent...

In any case, cutting of a country from the international payments system is about as hostile as a physical blockade or a siege. It's the kind of thing the US attempts to do on Iran, who could have predicted that the Eurozone would do it on an EU member state?

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman