Could you ever turn your back on someone that turned their back on you?

Easily ! Once they've turned their back on me, I don't associate with them anymore anyway! So I doubt it would ever happen.

Yes, and I have. If I give up on you, it's a serious thing. But they always deserved it. Other times I've shown true kindness, loyalty and unconditional love. As a life lesson. In so doing, I learned a lot too. One lesson was learning that I had the capacity to be a bigger person. That someone's ignorance or lack of care was not going to deter me from doing the right thing. There has never been a time that I regretted being the bigger person. Even if it was in vain, I was still proud of the way it was handled.

I have no problem forgetting they were ever born. It's only happened a few times.

I wouldn't need to.

They turned their back on me ... Thus there's no longer any relationship to turn MY back on.

If you're talking about a breakup then yes . Sometimes breaking up and turning your back on someone that you once loved  doesn't mean  you stop loving each other ( completely anyway ) it sometimes means it's time to stop hurting each other. 

Yep.

Yes, it's takes a lot for me to turn away, but once I'm gone, I'm gone.

One of my used-to-be best friends left me for the popular people, and I just can't forgive her for that unfortunately.

If their behaviour was harmful towards me then yes.