As to exactly how Greece can be forced from the Euro without a newly elected Greek government having a say in the matter, the decision could well rest with the group of men pictured below - the Governing Council of the European Central Bank.
It all depends now on Greek depositors. If the run on the Greek banks continues, the decision will land in the lap of the ECB, as Greek blogger Inside Greece explains;
To make up for the loss of deposits over the last two years, the ECB has allowed Greek banks, shut out from intermarket borrowing and lacking collateral that the central bank would accept, to be financed through emergency liquidity assistance (ELA). This means that the banks are able to borrow from the Bank of Greece, rather than the ECB, by putting up collateral that is theoretically more risky than bonds, such as small business loans or mortgages. It is thought that Greek banks have borrowed about 60 billion euros this way. But the supply of money is finite. Parliament has set the limit for the ELA scheme at 90 billion euros and Greek banks do not have limitless collateral.
Furthermore, there is the possibility that the ELA tap could be turned off if central bankers in Frankfurt become concerned about Greek banks becoming insolvent. In order to access ELA, currently the banks’ only source of funding, albeit a dwindling on, the ECB board needs to give its approval. But ELA funding could be halted with a two-thirds majority decision. This would cut off Greek banks from liquidity and Greece would be forced to begin printing its own money. Since it can’t print euros, the only option would be to return to the drachma.