Do you have proof they have resorted to crime?
What crime? Be specific. You could perhaps suggest slander, incitement to violence, or invasion of privacy.
High bar to satisfy a case for any of those three. Consider everything a private investigator could do IRL if one retained their services to track down an unknown individual; in general, none of those actions are considered criminal (though the spied-upon person would certainly wish the PI wasn't tailing them).
