YES! You've got it. Calmly walk away when she is barking. Do not reinforce her by yelling at her OR by reassuring her. Strangers are non-events for you; demonstrate that to your girl.
I would teach quiet like I would any other command. I would start in a place with fewer distractions and where you are more likely to get compliance. Maybe when you are out in the yard with her and she half-heartedly barks at something. As I said, though, don't think you are going to say quiet and she's going to instantly be cured. At first, you are going to reward one second of quiet (half second if that is all you can get). Then, two. Then, three. And on and on. Gradually increase the duration and gradually increase the level of difficulty.
As with other commands, you don't want to let her practice refusing the command. Dont' say "quiet" when you know she can't be! If kids are running and screaming close by, you know she's going to ignore you. Don't give the command then. Just walk her away.
Clear as mud?
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