I am probably not going to be much help but I remember DD1 going through a stage where she would hit things or herself when she was about that age. I had only ever smacked her hand once (because she was about to touch something hot and it was a quick reflex to smack her hand away, felt horrible for it). It did not last long though. We did what you have tried and even made out that she was hurting the objects she was hitting, in the end we started ignoring her because we noticed that she was watching us when she was doing it and waiting for a reaction.

Hope someone else has better advice for you.