I am with Bunnie on this one...Brutal Honesty. I will add this, reading your post, I can both relate to making someone dump you (I hate rejecting people, so like you I do something to make them toss me)....but what I have noticed through the years is the confusing behavior.
If you are shy and people don't know how you feel and they are trying to leave you alone "thinking you like her"....then she thinks that too. Men have always had a problem communicating, so they try to be nice (some women do too), but don't realize that by being nice it sends the wrong message to the person whose affections you do not want to solicit.
My number one advice here is DON'T TAKE TOO LONG to let your lack of interest be known, or you will risk hurting/confusing/pissing off someone who thinks: "... but he bought me a drink...."or "he smiled at me..."or "hey, everyone is leaving us alone, they must know something I don't...". Mixed messages are worse to overcome than coming right out and being honest.
To echo Bunnie....tell her she is a nice girl but that you are not feeling romantic chemistry and then do your best not to be overly nice because you rejected her, because you will confuse her. Don't be a jerk, but don't feel guilty and try to smooth things over by being too nice to her.