This is such a grey area and the concept of the governmental power (no pun intended) wielded through DEH is totally alien to the way electricity, a commodity, is supplied elsewhere in Europe that it is easy to make the wrong assumption but supposing it has to follow "European" rules then if you choose to take electricity from a French supplier then your contract is only with that French supplier. Yes the electric would be carried by DEH etc. as you say and theoretically they could come in and disconnect but then there would be an unsustainable legal position where a contract between 2 parties between whom there is no dispute can be affected by a third party (DEH) with whom there is no contractual basis. It would be a European legal nightmare effectively making the unusual (in Europe) Greek system effective across European countries where there are supposed to be no trade barriers. Whats also interesting is that the "French" supplier would be as powerful if not more powerful than DEH and they would have a vested interest in seeing DEH's power to interfere broken, something that cannot happen in Greece at the moment because the DEH is too powerful and too entwined in/with Government
Lets also understand that there are many Greeks too who would love to see the power wielded through the DEH reduced to that of supply and demand like other commodities.
Having said all of that a) it doesn't affect me and b) nothing in Greece would truly surprise me so it could happen that way