I can still see why people think she says "no", because at the end she smiles as she gets to the "P". But you can clearly see the "s" forming in her mouth, and if she was saying "no" then the way she starts the word would be completely unnatural. Also, look at the timing of her fist hitting the rail or whatever's below her. It matches with the "t" in stop, where someone would usually place emphasis (s-TOP!). If she was saying "no", then her fist slams down too late and doesn't make sense.