Partly a rant, partly a genuine question. DS is 2.5. Recently when he's annoyed with me or something I've done he comes over, holds my arm and smacks my hand saying "Naughty girl!". It's very deliberate. Clearly, this isn't something he's made up from his own head, he's copying it from somewhere. None of my friends smack their children. He wouldn't have seen his childminder doing it. My mum wouldn't do it either. The only other place he goes without me is to his Dad's (ie my ex). XP has a girlfriend who has a DD a few months older than DS. The GF and her DD live with him. So okay I'm jumping to wild conclusions assuming that either they smack her DD or they are using it for both of them when DS is there, but it's not a hard jump to make. XP was obsessed with smacking as a form of discipline when we were together and used to rant at length about how it was the only thing that worked and why were they trying to make it illegal and it never did him any harm. He also smacked DS' hand when he was 10 months old 
because he "pinched him" when he was crawling over him in bed one day. When I went mad at him for this he moved on to squeezing DS' hand tightly to make him cry but subtly so I couldn't tell he was doing it until he did cry. Again for spurious reasons such as "he scratched me" when he was about the same age and there was no way he would have comprehended what he was doing, let alone did it on purpose. Oh and 
I know this was awful but I did ask DS "Who hits you like that?" and he said "Don't know!" and I said "Does Daddy hit you?" and he said "Yes" - now I KNOW this isn't gospel from a 2 year old and he might say yes to anything but it still bothers me.
I know all the advice will be to talk to XP - but honestly there is no talking to him. Firstly I don't think he will back down on this since he felt SO strongly about it before, and secondly he hasn't reacted well when I've approached him about much more minor issues - e.g. DS was coming back with ribena and fruit shoots in his bag when he was 15 months old. He only drank water at home at this point. I mentioned this to XP politely and suggested maybe he could dilute the juice if he REALLY wanted DS to have it. He agreed all politely in front of my friend who happened to be there at the time and then sent him back the next week with fruit shoot again. Ie completely ignoring me and not even trying to hide it or acknowledge it saying "Actually I'll do it my way thanks". I mentioned it once more and his GF was there at the time and she gave me such a dirty look. So I left it at this point deciding there were bigger things to get upset about.
Also I have a feeling if I bring up something like this that either he or the GF will get defensive/aggressive over it - this happened when I queried his timekeeping. I don't want this happening in front of DS. XP never seems to come without his GF either.
And then finally I don't know if I even have a right to say anything - it's not illegal and he is his father. And obviously I have no say at all over how he and his GF choose to discipline their DD as she is nothing to do with me. Don't know if it's worth possibly creating more hassle for DS over it. I am currently waiting for an appointment with a solicitor so I can discuss my rights as I have some other issues with XP as well but nothing concrete so wanted to get advice before I did anything, but I'm told I won't get an appointment with legal aid until April now as all the funding will have gone.
But anyway, rant aside, any ideas how I should deal with this at home? Dealing with it in the same way as I've always dealt with hitting isn't working. It's like DS has an overriding view that he's in the right and if he's emulating behaviour from an adult he looks up to, how can I blame him? I have tried starting to say "We have no hitting in this house" which seems to make him stop and think a bit more, but I don't know whether that is enough - I don't want him to think that it's ok to hit anywhere else either. And I'd like to tell him that nobody has the right to hit him but I don't want to risk getting him into more trouble with his Dad thinking he's being cheeky etc. I could probably explain to him when he's a bit older that I don't agree with it, and that might be the rule at Daddy's but anywhere else it's not OK, but at the moment he wouldn't understand that.