When I started working my qh in round pen she had a nasty habit to kick out. Towards me. Well... When she tried this practice on me couple times she got smack on butt hard every time she turned it towards me. She got a point across VERY quickly.
So to answer your question, no, it's NOT acceptable. You can be hurt. First, you may consider a bigger area to work with the horse (even on lunge in big arena, for example), and second, you have to discipline it.
"Life isn't about waiting for the storm to pass: it's about learning to dance in the rain..."
"When we are no longer able to change a situation - we are challenged to change ourselves."
"How people treat you is their karma; how you react is yours."