In "Gunsmoke," Dick Sargent spanks Sue Ane Langdon in an episode entitled "Catawompers."
In "Bonanza," there were two (2) episodes: "Love Me Not" and "Woman of Fire." Anjanette Comer gets spanked in the first one. In the second, Joan Hackett is supposedly spanked, but since her face does not appear in the same frame as the tushy that's getting smacked, I suspect it's a stunt bottom.
There were also episodes of "Bonanza" in which children were spanked. Legitimate as these may be in context, there is no need to go into them here.