Both ECB and the Commission are way out of their mandated zones of action, so I do not think there is much they formally can do (like taking Greece to court). Informally the Commission can not do much if the Greece government is willing to use its veto to fight back. Maybe Greece does not get an EU institution, but that is about it.

There has been speculation as to what the ECB might do, which I understand as boiling down to messing with the monetary system. However, as long as the police and military gets their paychecks from the government, not ECB, there is in the end little ECB can do.

Just issue notes saying "this bill is good for paying 100 euros in taxes to the goverment of Greece" and stop paying your debts. First problem solved.

Since Greece runs (or did run) a trade deficit, some imports need to be slashed if trading partners are not found of collecting future opportunities for paying taxes in Greece. I would suggest starting with government imports of advanced weaponry.

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