Help I need advice

There is this girl who has crush on me and I don’t like her cause i already have a girlfriend and we’re trying to keep a relationship a secret because we don’t wanna make a big deal out of this but the girl who likes me won’t leave me alone after I specifically said that I don’t like her and me and my gf have expressed how much we don’t like her talking about it yet she continues to talk about it and I don’t know what to do