I'm thinking that it could either be a form of play for Emma or that famous "tude" that some kitties have. I know how much you love her but it's not fair to make the others miserable either. Have you tried the water bottle when she does this? Maybe you could give her a good squirt when she starts smacking. Don't say anything at all to her when you do it. That way she won't be sure it's you doing it and hold a grudge against you. Squirt her, and turn your head really quick and let her wonder where that came from. She, hopefully, will associate the water with smacking and stop doing it. I don't know if it will work but it might be worth a try anyway.
Good Luck!