For me it depends on the context. If they're some nutter preaching at me in the street they'll be ignored or get a "Fuck you!" from me. If it's just a kind old lady's way of saying thank you, I'll take it as such and just say "Thank you" in return.
The difference being, one is being kind, and the other is being a dick.
The difference being, one is being kind, and the other is being a dick.