What you said
Sanctions aren't always for the pupil being sanctioned alone. The rest of the class need to see that violence has a consequence or you may find that the autistic child isn't the only one hitting staff. The school has a duty of care to you, the teaching assistant and the other pupils.
Challenging behaviour is a common feature among children with autism as they can have difficulty with impulse control. This behaviour is often a sign that there is something wrong and the child has become highly distressed and anxious but can't communicate their problem. You may find The Autism Toolbox useful: http:bit.lyA94SuC
The expert view
Teachers have no right to refuse to teach, but the school does have a duty of care. If you are routinely assaulted - and that is what it is, assault - and the school knows about it, then the school is failing to provide a safe working environment.
I might add that they are also being totally invertebrate: it is easy for them to brush this under the carpet as they do not have to suffer the blows. You are going to have to step up your campaign and protect yourself, your teaching assistant and the other pupils from this child.
It may not be his fault. He may genuinely be beyond responsibility. But it is the school's responsibility not to expose you to danger.
Tell the head that, if the pupil is not given special provision, external to the classroom, you will consult your union as you fear for your safety. Your school is neglecting its duty appallingly.
Tom Bennett is author of The Behaviour Guru and Not Quite a Teacher. Post your questions at www.tes.co.ukbehaviour.