Sorry to burst your bubble, but no Elementals killed outside of the Elemental Planes stay dead. Hence Baron Geddon throwing bombs and Garr running amok together with Ragnaros. Ragnaros was only partially summoned into Azeroth in Molten Core, which left him weak, it wasn't what caused him to return.
Same goes for the Dreadlords, I believe? They can't be killed, or at least they didn't use to, permanently outside their own realm.