The problem is that simply batting her away has her thinking that you are playing with her ! So she bites more, if anything, encouraged by your response. Shouting 'no' at the same time sounds like an encouraging bark when she's in play mood !The best tactics have already been given. It's important that she knows to stop because she may continue what she thinks is play biting when she's older and possessed of adult teeth and adult staffie jaws! Some dogs never quite abandon play-biting or , the lesser, 'mouthing' when adults . I've a wolfhound who mouths,rarely but mischievously, but the important thing is she never hurts in doing it, it's just a gesture. You wouldn't want even that with a staffie !