How to get her to stop following us?

Ok so there is this girl. Lets call her Stacy. Stacy is really rude, stuck up, bossy, no fun, and wayy to boring and uptight to do anything. Well stacy used to be friends earlier this year with a girl who were gona call ann. Me and ann startef hanging out this year and she became good friends with my entire group of friends. Well everyone hates stacy but stacy cant cut the cord from ann who also hates her. When she didnt have a group to work with in class she cried, told the teacher we all hated her, and the teacher told ann that she would get a detention if she didnt invite stacy to b with us a lunch. So stacy wont stop following us around, and today we literally RAN away from her but she wont get the message!!!! We are in 7th grade by the way and we have a field trip to an amument park coming up and we dont want her following us around there either. We tried to involve her in convos before but it didnt work. Now we ignore her completley.