There is no legal basis for kicking Greece out of the euro. None of the relevant treaties have any provision for member states kicking another state out of the monetary union or a member state leaving of their own accord. I think there is a provision for a state to leave the EU, but only of its own accord and not by force, and they'd have to leave the EU not just the monetary union. So if its going to be legal, there'd have to be some amendments made to the treaties.

Of course, Greece could just turn its back on the treaties, ditch the euro, re-instate the drachma, convert all debt obligations to drachmas and go on its merry way devaluing the drachma against the euro. But they would no doubt face retaliation from an angry EU and will likely get hit with stiff trade barriers on Greece exports as punishment (in which case devaluing the drachma would have limited effect).

I think the more likely outcome will be some kind of debt restructuring, a situation where the foreign bondholders (who own almost 60% of the debt) will have to finally face up to their losses.