I'm not exactly sure when it started, but it turns out that one of the things that will instantly make me stop reading a story is spanking. As in, on the butt. Especially if it's depicted in a sexual manner, or used as public humiliation by the protagonist.
I don't think I can sensibly articulate why, but when it shows up it's an immediate nope. Maybe it's as simple as me being opposed to corporal punishment, but I can tell the reaction is visceral.