J.V.
Get some garage sale drums or create some out of buckets, oatmeal containers, whatever. Put em in the basement or wherever and take her there, saying, "We hit DRUMS, NOT people." and let her bang away. Sometimes playing a cd of beat-heavy music like rock can help her work out her agressive feeling by pounding away on the drums. Not only will it help her find an acceptable way to get her frustrations out, it might create a budding musician. An added bonus is that it will make her develop an understanding in rhythm and pattern, which will help her later in everything from sports to math!!