I would never ever slap or smack a puppy, ever. Puppies mouth, that's just the way it goes, I prefer a gentle way of training, there is never a reason for forcing your dog down or smacking them.
It takes patience and time, sometimes more with some puppies. Withdraw your hand, and say No firmly, leave the collar/harness and leash on and give a short quick tug of the leash just as she opens her mouth (don't jerk though). Make sure no one is playing rough with her or playing tug o war. Also, make sure puppy gets enough exercise, you already know that though. Be sure to praise when puppy removes teeth/stops mouthing.