Could you perhaps go visit this nice elderly lady, so that he becomes familiar with her and won't bark at her in the future? You could ask her if she would help you with this. Maybe she wouldn't mind. Perhaps all he needs is more socialization. Maybe when you have friends come over you could arrange a hallway encounter with them to help train him not to bark at them. That is all I can think of to do, other than working with him and saying no or carrying a can of pennies to shake when he does this.
