I would enlist the help of a trainer. The reason is that you may be creating superstitious fear/aggression with your use of the prong collar in that circumstance. "Hey look, new person, I'm worried, I'm looking at them, barking at them and then OW!!!!! screaming pain in my neck... NOW I hate them and am really scared of them because they made my neck hurt. I must get them to leave... etc"
Dutch Shepherd - Ptygo (Tee-Go) de las Flores