How do I deal with a girl like this?

There's this girl who has been creeping into my life lately. She's one of the most horrid people I've ever met. I've figured out she has stalked my fiance across 2 states to try to woo him. Well, he set her up (not realizing what kind of girl she is) with one of our friends and things are getting out of hand. She is leading him on. She'll only come hang out when she thinks my fiance will be there. She has tried to rip my fiance and I apart with remarks she has made. But somehow, neither fiance nor friend realize this. Another girl-friend of mine has met her and sees all of it. My fiance denies any of this is happening and says I must just be paranoid and I know he is genuinely not interested in her. But how do I keep my friend from getting hurt too badly and make sure this chick stays away from us? (Everyone knows I feel this way, but no one will listen. I've never been wrong about something like this either. No one seems to get that though...)