The following comes from section 805 of the Fair Debt Collection Practices Act. You can find the entire piece of Federal LEgislation on the FTC website:

http://www.ftc.gov/os/statutes/fdcpa/fdcpact.htm

(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.

If such notice from the consumer is made by mail, notification shall be complete upon receipt.

(d) For the purpose of this section, the term "consumer" includes the consumer's spouse, parent (if the consumer is a minor), guardian, executor, or administrator.