1. Open the skin modifier.
2. click on edit envelopes
3. make sure that the vertex radial button is selected.
4. select the arm bone in the bone menu.
5. select all of the vertices in the face and hair.
6. in the section that states 'absolute' enter in the number '0'. (This should make all of the weights for those vertices on that bone weight zero.)
7. while keeping the same vertices selected, select other bones (not the head bones or any bones that you want to act on these vertices)and make sure that the weight is '0'. This should help clear it up.