Okay normally, I think friendships should just fizzle out if that's what you want.
Since she's contacting you repeatedly and ignoring her doesn't seem to have the effect that you want, I think you should be direct. This means you either:
A) Tell her that you no longer want to be friends. You can couch this in a variety of ways, or tell her exactly what you told us here about why the friendship fell apart, or
B) Explain to her that you feel distant from her and explain why. If you go this route, you have to be open to the idea of coming to some sort of resolution and it will involve being open to hearing her side of the story and possibly becoming friends again.
I'd go for B in most situations, personally. If someone was a good friend in the past, chances are I still hold strong feelings for them even if things went badly. I also feel like if I'm upset with someone (or was upset with them in the past) they deserve to know why.
If you have a full life, don't want to deal with drama, or just plain don't care enough, go with A. There's no shame in that as long as you're not cruel.
As a random personal story, I was extremely close with a guy in high school. Yes, it was high school, but since then I have met few people in life that I feel as strongly about. Things went bad between us, he realized his behavior was causing him to lose my friendship, and he tried to change. But it was too late and I couldn't go back to the way things are. We eventually reconnected and are in each other's lives again. I almost didn't meet up with him that second time, but I'm so glad that I did. We're not super close and we don't talk often, but I've realized just how much I love him and how much he cares for me in return.
I guess what I'm trying to say is, you can't really go back, even under the best of circumstances. But sometimes it's still worth working through the bad stuff to get someone good back in your life. Even if it will never be the same.
