Be polite to her but don't engage her in unnecessary conversation. If you constantly ignore her, she may believe you are shy around her. If you spend too much time talking to her, she will certainly believe that you want to be around her.
Talk about the girls that you've liked before or even a girl that you have a current crush on. That should make it clear that you see this girl as nothing more than a friend.
Turn her down if she asks for a date or even an activity for just the two of you. Try not to be alone with her on purpose or she may believe that you are interested.
Offer to set her up with someone else. If you are actively trying to get her a date with someone else, she will be forced to realize that you don't intend to date her. You may even be able to get her to start liking someone else.
Tell her outright that you are not interested, if it comes to it. If she makes it obvious that she will not stop pursuing you, being honest is preferable to having to avoid her for the rest of your life. Tell her gently that you appreciate her as a person but that you don't have a romantic interest in her. Use positive language that avoids putting her down. The idea is not to hurt her feelings but to communicate your own feelings to stop the crush in its tracks.