ignoring "bad" behaviour. But what to do when ds hurts his sister?

[deaconblue]

We've been advised that ignoring "bad" behaviour totally is the best way to reduce it and that commands etc should always be voiced in the positive. But what am I supposed to do when ds(4) boots his little sister(2) in the stomach? Wondering if just picking her up and leaving the room is the way forward but I worry she will think we're not bothered that he's hurt her if we don't tell him off.
His paeditrician's idea is that a child's brain takes any kind of reaction from a parent as a reinforcement of the behaviour and withdrawing reaction makes the behaviour less likely to recur.

You are seeing a paediatrician so I'm guessing there are other problems?

If not at 4 I would say he is old enough to be told that he must never hit or hurt anyone and if he does it again he will lose a priviledge (something small that is a daily treat).

He has a variety of problems with social interaction. The doctor was very clear that the best way to prevent a behaviour was by giving it no attention whatsoever, we're keen to try this method and it has had positive results with tantrums but I'm not sure how to fit it if he hurts someone

I have my doubts about your paediatrician's wording/opinion but, assuming you want to keep to their advice...

Hurting someone else always gets you automatic time out when I'm in charge. And I don't mean fussing about for an hour putting a kid on a step and then running after them and putting them back again, I mean putting them into a room and shutting the door [holding it if necessary]. One minute per year of age, so he'd have four minuts. It completely cuts off contact with anyone else and while they do often get angry, they also realise pretty soon that they want to avoid it happening again. [N.B. I would not do this with kids who have issues about being left alone/in certain rooms etc.]

With all other behaviours that lead to time out I use warnings, but violence towards others is just an immediate removal. At four you know you shouldn't be hurting others like that. [However, if I do see them going to hurt someone, e.g. raising their fist, I will give a reminder then - "Just to remind you, you know if you hit them you'll be in time out."]

I don't know if it will work for you, but that's what I do.