[ Friends ] Open Question : how can i tell my friend enough is enough?

This friend works at same building, we been having few trust issue (i don’t trust her or her decisions about a lots of things had happened) she dose my heading but i am a very forgetful and forgiven person, she takes advantages i think without realising I know about it - I want to end this friendship but i have to be careful â€" mainly to avoid gossips at work, I hate letting people down but most girls in the past done that to me, i guess i never learn on my part - i like life so I go out a lot, I am very friendly, very open and giving person but sometimes people take advantage and let me down, when that happens I hate my self for letting it happen. How do I tell her that I don’t want anything to do with her in a professional manner? Points to add â€" she is not very matured person, never take advice, in denial about most wrong things she done and I think she has self steam issues â€" so there is no point on explaining why but I want to let her know I am not happy with her, she spoilt our friendship, she gossips about me but deny it, she go behind my back & cross the line between friendship the list go on. You may asked why I am friend with her â€" I didn’t know how bad she was and purely I felt bad for her cos of the past experience and most of all she was the only one who is friendly sounding my work place. thanks