I think it depends a lot on whether the slap is pre-meditated or a spur of the moment thing.
I remember that when I was little, I did something or other which was considered naughty - can't even remember what it was - and, after my dad had come home from work, and they had discussed my misdemeanour, they agreed that I should have a good spanking. this was some time after the offence, whatever it was, so Mum took me upstairs, made me lie down on my bed, pulled my pants down and gave me a good hiding. I was so shocked that I remember it to this day, as it was so cold and pre-meditated and so much time had elapsed since I did whatever it was. I'm still appalled at the thought of it.
On only about three occasions in her childhood, did I give my daughter a quick smack, and then it was immediately after she did whatever it was that was wrong.
The main sanction was a video ban or a felt-tip ban (!) if she was particularly naughty, and this worked a treat! (she was actually a very well-behaved child, but nobody's perfect!)