How to tell a girl you don't want to talk to her anymore?

Here's the story. I'm a guy that use to like a girl a few years ago and we always made this promise that we will be together or things like that. But as I grew older I start to lose interest in the girl and try my best to ignore and not talk to her. Basically staying as far away from her as possible. She's the type of girl that seek to much attention from other guys and the one that flirts to much. I delete her number, facebook, contacts, everything on the chart. But as times goes by she keep calling, txting, and emailing me over and over asking why I wont call her or talk. Im getting a little annoyed and going to college in a few months and dont want to talk to her ever. And i mean ever again cause I see that shes not really worth it. I'm a nice guy and want to be sincere and nice as possible when telling her I dont want to talk to her ever. She have a boyfriend and still wont leave me alone with this love thing she thinks I still have for her and seriously try to call me and leave me messages in the middle of the night. So what should I say to her.