Luke Skywalker said:
Reverse psychology is the answer. I am master of getting girls to reject me so I'm the best qualified to give advice here.
Make excessive compliments, call her every day tell her you cant stop thinking about her, supplicate to her, ask her if she likes you, behave really wussy, etc... and act like really needy, just as you would for a really hot girl that you are really interested in who is giving you an adreline rush, and pretend that you'll be crushed if she rejects you, and she'll be running for the hills, guaranteed.
Definetly will work. However, I wouldn't recommend doing it because it may ruin your future game with HB's which you do want to score with. My philosophy is that every fat chick somehow knows an HB9 or is linked to one in a social chain. Therefore, word gets around quick that you are a desperate/needy person.
Best thing to do, ignore her. She'll get the message, and if she doesn't, it's unfortunately too bad for her. You didn't lead her on (excessively), or tell you that you like her (or give any hint that you may go into a relationship with her), so she has no reason to be mad at you. If she's a very outgoing girl and somebody who you feel you can be honest with, tell her the truth! (And no, I don't mean tell her she is fat.) Rather, say that you feel you two are two seperate people, and you don't feel that you could go into a relationship with her.) However, heads up, most girls aren't really the second type so just ignore her. Problem solved!
Oh, and one last thing, ignore her over a period of time. Therefore, if you used to talk to her 5 times a week, next week lower it to 4 times, than 3 times, than 2 times, than none. (Or something a long the lines of that.) OR, if you used to spend 1 hour a day with her, gradually decrease the amount of time you spend with her as time goes on. She'll eventually become disinterested.
Hope I helped,
Axcell.