If you don't know them tell them to fuck off!!
You can either be a good samaritan or a dick about it. Some people may need a stranger just to listen but we all have to remember we all got shit goin on, that's why we have friends. And you can definitely tell the difference between a genuine person in need and an attention seeker or even just one of those people that talk to fucking anybody and everybody!
Society has far too much people pleasing going on. If you can't be arsed, just say. Who cares what they think, you don't know them.