My DD doesn't do this now, but when she did I would say 'We don't hit, that hurts Mummy, I want you to say you're sorry now'. I would get down on her level and look her in the eye. Once they get the message that it is the wrong thing to do, getting eye contact can be more difficult! But I think I am reinforcing the message that this is not the way to respond if we're cross. As they get a bit more capable of listening, telling her that she doesn't like it when [a friend - use their name] hits her, does she? I try simple explanations.

I think as long as you continue to reinforce it, the message gets across. They are still learning to control their emotions and will still slip up for quite a while.

If a toy is involved (throwing, hitting with the toy) I tell DD that if she is not going to play nicely with that toy, then Mummy has to take it off her and put it away. And I do if I need to, every time. I don't have to do that very often! One warning is generally enough.

I think doing anything physical back to them is completely the wrong message (and certainly not gentle parenting). You are as an adult then lashing out at them and expect a toddler not to do it! All that says to me is, you're not allowed to do it, but Mummy/Daddy can.