Send the original to the collection agency; a copy to the creditor; and a copy to:
Federal Trade Commission
225 Peachtree Street, NE
Atlanta, GA 30303
Send these letters certified mail, return receipt requested. Keep the third copy of the letter, the Certified Mail receipts, and the green cards (which you should receive back in the mail) for your records.
The collection agency should then stop contacting you except to provide information about their basis for claiming that you owe a debt. If they continue to contact you, make an online complaint to Federal Trade Commission.
How to Fight Back Against Collection Agencies – older article, not sure if it current and/or accurate.