Is it ever acceptable to ignore someone?

The church youth group I went to was fairly typical as far as the kids who attended.  There was one girl in particular that no one seemed to like though.  I couldn't stand this girl, and most likely made it very clear on more than one occasion. The girl (I'll just call her C), would drive people crazy.  At one point, my sister was a youth leader and started talking to C.  C then gravitated toward my sister, always wanting to be around her or talk to her during youth group, and calling her about 20 times a week.  It finally got to the point that my sister would check the caller ID on her cell phone and if it was C calling, my sister would quickly open and close the phone, to cause the call to hang up. Apparently this is common behavior for C as I have heard of her doing this with other people as well.  Looking back, I believe C may have some form of mid-functioning autism.  I'm basing that on what I know about autism and the different behaviors I have seen from people on the spectrum, not on any kind of medical proof or information from a doctor.  Regardless, C almost certainly has some mental disability in one form or another. These days, C attends the same school as I do.  She rides the city bus, but not the same bus as me.  I quit riding that bus when I realized C would be riding as well and take a different bus to school.  I sometimes hear her yelling across the table to people she knew from high school.  I have been known to leave the cafeteria if I hear her talking.  I either leave the cafeteria or roll my eyes while thinking Shut up, C.  Seriously, just stop talking. I have warned other girls I know to steer clear of this girl and not give her their contact information.  Really, I tell girls not to talk to C, as I believe not talking to her is the best way to handle the situation.  When I was talking to my brother-in-law, he agreed with me that ignoring C is the best thing to do because of the situation and previous experience with C. What do you think?  Is ignoring her and advising others to ignore her the best thing to do, given the circumstances?  What else can be done in this situation?