You should sit her down and explain to her that touching (and point to the area but don't touch) is meant for only in her room and not to be done other then her room. That sounds odd though, is their anything wrong with her? Was she sexually abused, I am asking cause I was, I acted like that when I was at 8 (you know humping things cause I couldn't get rid of the feeling of wanting you know sex) I stopped that but with me I didn't really do it in public. So the other question is if something is wrong with her and if not then I would seek out help, if doing what I think you should try isn't working. That is not good she is doing that in public, it could attract disgusting perverted child molesters, its not good. If I offend you I am sorry, just need more info cause what I asked is related to trying to answer your question.
Treat it the way you handle correcting any other undesirable behavior. Warnings, consequences and rewards for refraining. Just because it's touching the vaginal area does not mean she wouldn't understand "no."