How do I stop communicating people that I don’t want anymore in my life?

We are social beings you know..... Its true that we should get rid of toxic relations asap for our own good...... But as for your situation, things are a little different. You had a crush on her,but now you are over with it and you don't wanna communicate with her anymore. I understand that talking to her makes you uneasy or brings back 'not so good' memories.... But you know, she doesn't know about any of these.... So she still keeps in touch with you for the sake of your friendship. As @Mechalith said, maybe you can approach her, if the matter is truly getting to you then talk to her..... worst case scenario, she freaks out and never talks to you again, which wouldn't be much of a problem for you....
Still, i would like to say, everything in this world isn't sweet, and we need to accept it. If we do not like someone, we should not waste strong emotions on them...like hate, love or anything.... If you have nothing to do with someone, they shouldn't bother you, but stopping all sort of communication with them can result in a long term loss..... However, best of luck :)

Something along the lines of "Look, you're a great person, but it didn't/can't/wont/etc work out between us, and I can't handle being 'just friends'. I think it's best if we don't spend time together anymore."The longer you avoid the subject, the harder it's going to be.

@ Voilet, thank you. You give the best advices ever. :)