Just send the following drop dead letter to citibank and by law they need to stop calling you.
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CitiBank (South Dakota), N.A.
Customer Service
P. O. Box 6500
Sioux Falls, SD 57117
I have been contacted by your company about a debt that you allege that I owe.
I am instructing you and/or any company representing you NOT to contact me any further in connection with this alleged debt.
Under the Fair Debt Collection Practices Act, a federal law, you may not contact me further once I have notified you not to do so.
I am hereby advising you that continued contact by your office will cause severe and/or permanent harm to me. I suffer from a number of medical problems, both physical and mental.
Sincerely
John Doe
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Only include the last paragraph if it apply to your situation. In my case it did.
Send this letter certified mail with return receipt. Also, don't forget to include your name and account number.