Every once in a blue moon, maybe 2-3 times a year, a girl will chat me up somewhere random (bars, in the street). This is rare enough to feel special every single time, and I highly respect it when a girl musters up the courage to do so.

When I'm not interested, though, eg. due to lack of physical attraction, I usually end the conversation pretty soon in a friendly manner and keep going.

Are there alternative ways to act in these cases, i.e. where you would spend some time with the girl right then (if they're down), or take their number anyway, that would be worth the obvious downside of having to communicate the disinterest at a much more incovenient later time?