You can seek a civil harassment injunction to stop them.
You need to complete forms N1:
The Court will set the matter down for hearing and decide whether to order they stop the harassment.
If the Court does and they refuse, this could be contempt of Court which may be warned, fined or sent to prison.
The Protection from Harassment Act 1997 says:
1 Prohibition of harassment.
(1)A person must not pursue a course of conduct—
(a)which amounts to harassment of another, and
(b)which he knows or ought to know amounts to harassment of the other.
[F1(1A)A person must not pursue a course of conduct —
(a)which involves harassment of two or more persons, and
(b)which he knows or ought to know involves harassment of those persons, and
(c)by which he intends to persuade any person (whether or not one of those mentioned above)—
(i)not to do something that he is entitled or required to do, or
(ii)to do something that he is not under any obligation to do.]
That should resolve it for you.
Can I clarify anything for you about this today, please?
If you need anything further I am available for a follow up at no extra cost.