§ 1692 c (c). (c) CEASING COMMUNICATION. If a consumer notifies a debt collector in writing that the consumer refuses to pay a debt or that the consumer wishes the debt collector to cease further communication with the consumer, the debt collector shall not communicate further with the consumer with respect to such debt, except –

(1) to advise the consumer that the debt collector’s further efforts are being terminated;
(2) to notify the consumer that the debt collector or creditor may invoke specified remedies which are ordinarily invoked by such debt collector or creditor; or
(3) where applicable, to notify the consumer that the debt collector or creditor intends to invoke a specified remedy.

your name
your address
your phone # (optional)

today’s date

collection agency business name
address

RE: Account #_________

Dear Sir or Madam:

I request that (collection agency name) CEASE and DESIST immediately in its efforts to collect on the above referenced account. I will not deal with any collection agencies and I will only deal with the original creditor of this and all accounts bearing my name.

Furthermore, (collection agency name) and all its agents and assigns are hereby instructed to immediately CEASE and DESIST all contact with myself and with third parties such as my family members, employers, coworkers and the like, via telephone, including but not limited to computer generated calls, and via person to person contact at my places of residence and employment.

Any further communication with me regarding this account or any accounts bearing my name or phone number are to be conducted ONLY in writing via USPS to the above address.

Please attend to the actions required by your receipt of this letter immediately or face legal sanctions under applicable Federal and State law.

Sincerely,
your name

The harrassmentb rules are spelled out pretty clearly.