What is the easiest way?

Easiest way to forget about a girl is to find something constructive to do with yourself that will take your mind off of the world. Exercise, hang out with your friends, do random fun stuff: go rock climbing, go biking or swimming, read a book (try the divinci code or enders game) distract yourself by whatever means possible. That should work pretty well. Good luck

This is a little hard to answer without you being more specific with details of your problem, but short answer, just say good-bye and move on. It doesn't sound like she is worth your trouble.