What are you hoping the letter will say?
Why don't you just ignore him?
It is not quite clear what specifically he is doing.
Ok. So what is he specifically doing?
I'm really sorry but I'm afraid he cannot be prevented from doing that.
That is not harassment. That is just general conversation.
It would be different if he was contacting you or asking them to make adverse contact with you. He is not. You cannot lawfully ask him to stop mentioning you ever in conversation with anybody.
The reality is that a harassment warning could not be effective here because he is only doing something he in lawfully entitled to do. I'm really sorry but this is just plain not unlawful. I understand it might be annoying but no more than that.
Can I clarify anything for you?
Yes, but this isn't coercive control. Coercive control is a high test anyway that won't have much application.
This is just mentioning you to others. He is allowed to do that.
The police won't be interested in this because it isn't unlawful.
Also, you need to be careful about making allegations over things like this. The case of Waxman effectively says that making unmeritorious allegations is harassment in itself and you don't want to give him a cause of action against you.
If you have tried to report this to the police and follow up with sending him a harassment warning and he came to me for advice I would be telling him to sue you for harassment on the basis that you are making vexatious allegations.
Don't give him the satisfaction.
Yes, I understand but on the facts you are presenting so far you would be at risk of a harassment action.
One report to the police wouldn't be sufficient as it is just one but if you followed up with a further act of vexation that probably would.