In 5.5 years we have not found sufficient cause to use spankings, we cannot think of anything bad enough that the proper answer would be to hit him.
He has pushed our buttons, and I have found myself more forceful than I should have in moving him along or putting him into his room, and I've always ended up feeling bad even about that.
Apropos of nothing, my wife told me just a few years ago that she still flinches when I take my belt off when getting undressed, not from anything I've done of course, but from her punishments as a child. A thirty-mumble year old woman still flinching? Anybody want to say that doesn't cause long-lasting harm?