JohnS47:
Goodgulf, the movie was Walk On The Wild Side.
I went looking. I found many sites that said that scene had been cut from all known copies of the movie. The only "spanking" bit left in the movie is the bit where the whorehouse bouncer bends a girl forward in bed and beats at her. It's a hard angle for him to hit anything except the small of her back.
Those talking about the missing scene refer to it as a great lesbian spanking scene - which might be why it was cut in the mid 60s. Not because of the spanking aspect, but the lesbian ones. Making the lesbian undertones of that movie (which were pretty explicated for the day) too obvious.
It seems impossible that a scene could vanish from a movie like this, but there were all. Sorry, but know that I envy your memories of that scene.