Can you carry a stick and stay close to him?  A golf club is also an option as it is light and yet the club head is effective.

I would ask the owners to please control their dog(s).

P.S.  
I personally would never beat a puppy.  Easy enough just to grab them by the collar and pull them off and slam them on their back. It shocks them enough to back off.  But I am strong man with MMA training and a lot of dog experience.  Hand fighting a dog is not much different than a human if you know the moves they use. I have broken up 3 dog fights and 1 attack on human with my bare hands. If they might come back then as soon as they are on their back slide your other arm between the front legs and grab them by the throat or collar.  If they squirm take what is called full guard (you are going to get scratched).