How do you stop someone from ruining someone's life and the people around them

Because this person has a crush on someone's boyfriend and tried breaking them up didn't work and because of her coursing trouble for him they are no longer friends she wants to destroy his live and course trouble for him and lives of others and getting your children involved to get them to there house and you have had to tell your children to stay away .how do you deal with and sort it out if other people are also involved getting info for that person and being so sly what can you do about it with no proof or evidence.