I have heard of alot of head bangers & my husband actually used to hold his breath to scare his mum. They all said the same. As hard as it is walk away & ignore it.She is only hurting herself & soon she will realise that not only does it not work, but that it hurts!
My mother in law was so scared she took my husband to the doc & you know what he said? Let him do it till he passes out, not only will his body start breathing again when he does, but it will scare him out of it. Well he only did it once!
It will be hard, but you need her to know that no matter what she does you are boss. She will not get her way no matter what. It sounds harsh doesn't it. But that is how my friends overcame it.
It's up to you if you listen to my advise, but either way good luck & I hope it doesn't last too long!