If you know anyone being harassed, or are being harassed yourself, then you should report it to the police as it's a crime to "cause fear, harm or distress".
If it's the first instance then it won't be a full crime & they should be issued a warning letter that if they continue with their course of conduct then they will then be guilty of the full offence and prosecuted. If it's a report of an ongoing campaign and you have evidence to support the allegations of it being more than a one-off then it will go striaght to full offence scenario straight away.
cheers
FF