How do you put up with people that upset you by just looking at the?

We used to be friends but she crossed WAY to many lines, and she went so far over them it's unbelievable. I get angry and upset just when I see her and I can hardly muster up politeness. I really don't want to start something by so obviously not being able to stand her. Is there any way that I can get the daggers to stop shooting out of my eyes or do I just hope she doesn't turn it into something?