I occasionally give my son a sharp tap on the side of the head. I have 3 kids and he is the only one I do this with. Words do not work with him, at least at this age.
This is very close to being illegal where I come from.
I'm careful to do this only when he's say, throwing things recklessly or otherwise engaging in behaviour with real risks to others.
It's not a "clip round the ears" and it's not meant to hurt. It's also not intended as "punishment".
What it is though is a reliable way to get his attention; being honest he just responds much more readily to this than words as a way to communicate "stop now".
Sample size n=1, all families, situations and children are different.
As I write this and think about it I realise there is probably a better way to do this, all suggestions appreciated.
EDIT: previously I found myself yelling, using "stern voice" or physically restraining him before I got to the point of thinking about a "physical signal".