Talk to someone live, ask for their name, their agency name, address, phone number, the name of the debtor, the name of the creditor, and the date and amount of the debt. (They probably won't tell you most of the last information without confirming you are the debtor but ask anyway). Then say that the debtor does not live at your address and that you know your rights under the Fair Debt Collection Practices Act and that you will sue them if you receive even a single additional phone call. Follow up with a letter, then sue if you receive any further communications.