You should arrange for a meeting with her. Make sure that one of your trusted friends is present to avoid ant form of deniability on her part. Clearly tell her that she should not contact you and there is no chance that you will go out with her. Do not offer to be friends with her because she might use this to be always close to you.
tell her something that will totally turn her off , discust her and have really boring conversations . if that doesnt work tell her to back off . if that doesnt work.. try to be as ugly and annoying as possible and start doing what she does to you . if none of that works... i guerenteee .. if you are with her and she annoying you so badly , give her a really weird look and just stand there not moving at all . i know its weird but it works ! as long as you dont care she thinks your the weirdest thing she ever met in her life it would work really good ! she would probably laugh at first but then she would get mad and then.. she will think your weird and leave you alone ! (: hahahaha
ugliness is not just the thing and either its not her fault.. you should first try to know whether she is that much serious and really loves you so much..then talk with her with calm mind and do not disappoint her at last how much ever she may be ugly she is also a human being and has right to love.. she loves you that's not at all big issue talk with her tell her you are not interested right now... if she really cares for you she will surely understand do not be rude or say something bad out of frustration just tell her you are her friend and only friend that's it... and you are not interested into her the way she thinks... may be after this she may try to come close you keep on emphasis on just being friends and the fact is she will understand it....remember not everyone has a face of an actress some are lucky..... but do see her character first as physical ugliness can go by lots of things but ugliness of behaviour you have to face for whole life