IMO (and indeed IME) the 'absolutely not!' (or in my case the 'NO!') isn't a further resort after the usual stuff has failed, it's for different situations - hitting, running across roads, child about to go headfirst down stairs.
think of something else for him to do, pref in another room away from said socket, even if it's helping you wash up
or if there is nothing more appealing elsewhere leave the room with him anyway and go somewhere boring with the reasoning 'But DS, I asked you nicely to stop playing with the socket, and you kept on doing it, so we can't stay in here any more, we have to sit in the kitchen' (or wherever)
I do this with the tv - 'But dd you fiddled with the buttons, so the TV goes off'
NB if you do this you must carry it through, at least temporarily. Do not be swayed by howling. But after 10min if he says (calmly and reasonably) 'more tv?' you can say 'yes more tv but no fiddling'