Most Helpful Girl
You just don't interact with her. Simple.
Most Helpful Guy
First question you need to ask yourself is, "Do I want her attention and/or friendship?" If the answer is no, then you've solved the problem. If you want this person in your life, you have several options:
1) Establish clear and firm boundaries for you friendship including limits on acceptable behaviour;
2) Communicate these clearly and specifically to this person and finally, speaking of communication;
3) Attempt to talk to her about issues that are bothering you and how they impact your friendship with her.
If she genuinely wants to be your friend, then she will take into consideration your thoughts and concerns and adjust her conduct accordingly. If she doesn't, then you need to let her know that you cannot tolerate things as they are and stop associating. Let others know as well and prepare to deal with any rumours or gossip she may be spreading.