Well, my philosophy has always been that most friends/acquaintances enter your life for a specific time -- usually to fulfill a certain need at a certain time, if you follow -- and when that's done, or your life is different, it's time to move on. It doesn't necessarily mean they're a bad person, or you are, or anything, just that your lives no longer intersect the way they once did.

And in this case? Not returning her calls and slowly but firmly distancing yourself is probably the kindest way to go.