Spanking is a hard subject for me because I grew up with abuse - when my father "spanked" me, it always involved bruises and sometimes bleeding.
I learned later on that spanking is not what he was doing, obviously.
So I'm leery with spanking - but I have done it on rare occasions such as when my daughter ran to the road and almost crossed it...and when she started to play with the stove. She was barely three but she STILL remembers those moments (she's six now)...she never ran across the road or played with the stove again.
My second daughter has a little stubborn streak. She's way too young to be spanked (I think they need to be at least three, and the spanking should never leave any kind of mark - it is more to shock and make a point than to hurt). Also you should never use any objects such as belts, spoons, wires, etc. I only use my hand. I'm good at keeping a low, calm tone and neutral facial expression when I explain why I do it, etc...but later I always, always cry when my oldest was not in the room. But I rather her experience a little shock and sting than to be hit by a car or burned by a stove...