Unfortunately/fortunately, petitions usually have very little impact. In this situation, it's likely only garnering him more views. I agree that those that he has slandered, should try and put some sort of lawsuit together. Considering he has defamed many individuals, they may actually have a strong case.
Once an individual has been accused of heinous acts, it's going to stick with them, regardless of whether it's true or not. People are quick to jump to conclusions.