When the demand for loans heats up, macro-prudential authorities can decrease the loan-to-value ratio, thus increasing the cost of credit and slowing down or stopping its growth. Both counter-cyclical capital buffers and loan-to-value ratios increase the cost of credit and thus limit its expansion, as does an increase in interest rates.
As soon as possible, as there is a value in not waiting too long. The Greek economy has the potential to rebound when depositors bring their money back to the banks and banks can again lend to the economy, and when foreign investors come back. But of course all of this assumes that the Greek government delivers the agreed reforms.