I would say to her quite sternly that "In this house we use our words not our hands, if something is upsetting you tell me what it is do not hit mummy or smack or you will go straight to your room" Then if she does it again take her straight to her room, no warning. Then whilst she's in there explain again that she is to use her words not her hands, and that when she can apologise she can come out. Then when she does get upset about something but tells you rather than hitting you praise her. Whilst Paris has never been a hitter, a gf of mine did this and it worked well, it took a while for her child to understand the consequences of their actions but once they got the hang of it it worked wonders. I wouldn't be smacking for smacking as I think that sends the wrong signal to the child, but I am not a smacker at all so thats just my way of thinking.

Goodluck!

*hugs*
Cailin