Once a Debt Collector receives your Cease and Desist letter, they are required to stop all communication except to notify you about the actions that will be taken against you. Sending the letter does not mean you don't owe the debt!
Send this letter by Certified Mail with a Return Receipt Requested.
Dear Debt Collector:
Re: Your File #
Name of Creditor
I dispute the validity of this debt or I refuse to pay this debt.
You must cease communicating with me in reference to this debt.
After you have sent this letter, and you have received the return receipt and they are still calling you, then you should give us a call.