Princess Kira did not want to deal with anyone who was unruly so she chose to spit on the guy and ended up making him regret what he had done. He was humiliated because he was unruly and because she had never punished him before, she had to start with something which she considered easy and no painful. So she used spit fetish. And she warned him that if he did not learn his lesson, she would do something worse to him.