Do a Putin on their asses.

As long as the assets have not left Greece (and the more valuable ones cannot leave Greece), it is a trivial matter to send goons with guns and reclaim them.

Cite force majeure, bribery or even simply compelling national interest.

Less provocatively, tax them until they become unprofitable, then offer to buy them back for the same nominal sum they were sold for, minus any deferred maintenance.

Really, the number of ways to renationalise things you need to renationalise in more or less legal ways is legion.

- Jake

If you only spend 20 minutes of the rest of your life on economics, go spend them here.