What could ECB and or the eurogroup do if it really, really wants Greece to start using something other then euros?

Greece could adopt currency controls and the Greek Central Bank could seize all of the Euros it could and then use these as the backing for an alternate, internal currency, electronic, paper or both. That is the system on which much of the US banking system operated from the US Civil War until the Fed came on line in 1913. Then the ECB, IMF and Eurogroup member states could actively facilitate 'market attacks' on this new currency to drive it away from par with the Euro, which would not be a bad thing for the Greek economy unless they managed to get the currency to appreciate.

Meanwhile Greece would formally remain in all of the various governing boards in the EU and Eurogroup and be able to vote and veto as appropriate. They could make life miserable, if not as miserable, for other states as well as for Greece. At some point the Russian card might well get played, and just blocking certain actions that adversely affect Russia would really upset some in the Eurogroup as well as the USA.  
 

"It is not necessary to have hope in order to persevere."