Hitting

Guys, my sweet angel has entered the dreaded "hitting" phase. It's not even when he's mad (although he's done that from time to time) but it's mainly just "for fun". He's whacking us, the dog, anything he wants. And it hurts! lol what are you tips and tricks to get him to stop? I'm afraid that our "reactions" are just what he wants, but it also feels odd to just ignore it and not have any reaction.