When my son refuses to heed my "NO" and especially when he acts as if we're playing (and this usually involves him and an electric fan or sockets or the TV or PC), I put him in his pen and let him cry.
Of course, since there are toys in the pen, he sometimes ends up playing but it's usually only a matter of minutes before he realizes his space and movement has been limited. So almost always, he ends up crying and I let him be.
For a while.
And though I pick him up, hold him, pat his back and wipe his face and brow... though I tell him I love him and give him kisses, I keep on scolding him.
They say consistency is the key. I should remember that, because sometimes I just lightly slap his hands while giving a stern "NO" and wagging my finger. Maybe the timeout in the pen will work better (but what if the pen isn't around to restrain him?).
Or maybe I shouldn't be so impatient. After all, kids will test your "No" evey step of the way. That's how they learn to depend on you too.