If a person does not understand your intentions, you have to make them understand. Talk to her about your engaged status. Clarify that you are not interested in any relations with her. This should stop her from taking interest in you, although it may take sometime.
If she does not withdraw her interest then possibly she’s having problem of emotional dependency, she may be developing erotomanic delusions (pathological love) towards you.
Bottom line for you is, after clarifying your stand do not pay attention to her, do not try to convince her and do not agree for friendship, which may cause emotional issus in future.
I am a 28-year-old male, working as a freelance photographer. I am engaged to a nice girl, but one of my colleagues is madly in love with me. She never hesitates to show her fascination for me. How do I put her off? Why canâ€™t she understand that I am not interested in her at all? How do I make my intention clear to her?