Most Helpful Girl
First off, you don't use the words "Most likely." That tells her that you're interested and she needs to make you MORE interested. It tells her she has a chance. You have a girlfriend; she has no chance.
You cannot avoid hurting her feelings. She needs a wakeup call, because what she is doing is disrespectful to you and your girlfriend, not to mention hurtful toward your girlfriend. You need to tell her straight up. "We are not dating and we never will be. STOP telling people that." Don't call her names or anything to be cruel, but don't be afraid of hurting her feelings. BE FIRM. If she doesn't stop, you have to stop talking to her and seeing her.1THIS IS NOT RELEVANT ANYMORE