Someone You Need to Let Go, or Wish You Did Not Know

This one is difficult, because I literally cannot think of anyone. You see, when a friend or acquaintance becomes somebody I do not wish to associate with any longer, well - I stop associating with them. Many times I do this prematurely and later regret it. Sometimes the damage can be mended if I swallow my pride and apologize, and I have done this more times than I want to admit. But it only very rarely happens that I retain the companionship of someone that I feel is bad for me in some way. The reason for this is simple: life is short, and I do not need to waste precious mental and emotional resources (which are scarce) trying to maintain a friendship that will only bring me down.
Now, for the other part of the question...I do not know anybody that I wish I did not know. I am the sum of my experiences, and the people I have come across in my life have all done their part to contribute to those experiences. Some have contributed negatively, some positively, and some (the vast majority) have done both. No, I would not change the people I have known, not for the world.