A whack, in the corporal punishment context, is a stroke with an implement (e.g. I got six whacks with the strap for cheating). Whack is also an onomatopoeic word which aptly describes the sound of a hard flat implement striking the buttocks or the palm of the hand hard.
To whack is the corresponding verb. A whacking is a hard spanking with an implement.
Thwack is possibly an even more expressive word, incorporating as it does the slight hesitation and build up to the explosive, percussive sound. On the other end of the severity spectrum is a light "pat" or "tap".