Come on. You take the hardest object you can find, throw it at her and yell “Shiksa” at the top of your lungs. That alerts the tznius patrol, who deal with her as a pritza should be dealt with!
Sometimes looking the other way, as I once did when a woman (who turned out to be my friend’s wife) is improperly dressed. She got the message judging by her reaction (and fortunately she did not see who it was because it was dark and I was wearing black).