The federal Fair Debt Collection Practices Act regulates communications in connection with debt collection. In particular, section 805(c) of the Act states the following about ceasing contact:
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. If such notice from the consumer is made by mail, notification shall be complete upon receipt.
There are many sample "cease contact" letters for use with third-party debt collectors. For example, the Consumer Financial Protection Bureau (CFPB) provides sample letters you can download. Also see this publication from the National Consumer Law Center that contains a sample letter [PDF]. Also see our research guide on debt collection.