This letter is not a joke. It’s a template that can and has been used successfully (in England).
I refer to your letter dated the xxxxx in which you allege there is an outstanding balance of £xxx owed by me to your company.
I do not acknowledge any such debt to your company.
I ask that you no longer process my data and that you cease all further communications as is my right under Section 10, subsection (1) of the Data Protection Act 1998.
You have 7 days from receipt of this letter to state that you comply or intend to comply with my request. Should you not comply or intend to comply, you are to state your reasons as to why.
Please be aware that I will regard any further approaches other than in writing as harassment under S40 of the Administration of Justice Act 1970, and any inappropriate written contact as harassment under that Act or S1 of the Malicious Communications Act 1988.
As such you are now to cease any telephone calls to my residence or workplace. Furthermore, should it be your intention to arrange a “doorstep call”, please remember that there is only an implied license under English Common Law for certain people to visit me on my property without express permission; the postman and people asking for directions etc (Armstrong v. Sheppard and Short Ltd  2 Q.B. per Lord Evershed M.R.).
Please therefore take note that, I revoke license under English Common Law for you, or your representatives to visit me at my property and if you do so without my permission, you will then be liable to damages for a tort of trespass. You would also be conspiring in a trespass if you sent someone to visit me nevertheless.
Should you decide to ignore or continue to contact me about the alleged debt, I would ask that you provide documented proof that
a) the debt exists and
b) you are acting legally in your instruction for the alleged debt.
Failure to comply with any of the requests in this letter will result in further action being taken by me. This may include and may not solely be restricted to a complaint being made to The Office of Fair Trading and the Credit Services Association with regard to your professional conduct.
Please note that you will receive no further correspondence from me on this matter. The only communication I would now expect to receive from you would be confirmation that the matter is now closed. This matter is not going to go away and ignoring the problem could potentially make your situation worse and I therefore strongly recommend that you heed the contents of this letter.