When you understand that ALL puppies bite and nibble as a form of play the answer is not punishment. Gently grab the puppy's lower jaw, in the front at the chin, with your thumb inside and forefinger underneath. Squeeze slightly and give whichever training command you wish to associate with that issue, do NOT shout. I use "Careful" and then allow them to nibble, as long as they are being gentle, so as to not eliminate a natural instinctive behavior. After they have learned not to bite hard this command word can be used to modify ANY rough or aggressive behavior if used in a proper conditioning pattern.
My father worked as an animal trainer in the 30's and as a K-9 trainer for local police in the 50's. Have raised dogs all my life.