nor am I knowledgeable about UK telecommunications rules and practices. But unless this person has criminal intent, the "Misuse of Computers Act" does not seem to apply when I reviewed the outline of it.
If you have documentation of all IP's and log in information. I would go though the ISP's he uses and lodge complaints they have a person causing disruption and is using their service not in a manner that abides by the ISP's TOS. Then if the ISP's ignore you, then you need to consult with a lawyer and look into your recourses with the ISP's.
It's a pain, but thats the direction I would go with it.
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