Ok - you can take a civil action.
You can seek an injunction to stop the harassment if the Police do not want to do anything.
You need to complete forms N1:
The Court will set the matter down for a hearing and decide whether to grant an injunction.
This is harassment as it is pursuing a course of conduct likely to cause alarm, harassment or distress.
If the tenant breaches the order then this is contempt of Court, which he can be warned, fined or sent to prison.
But based on what you have said, I think that you will get the order.
Can I clarify anything for you about this today, please?
If not, I would appreciate a 5-star rating for my answer. If you need anything further I am available for a follow up at no extra cost.