The two things you can do when a debt collector keeps calling you on a debt you've already paid for or settled is either send a cease and desist letter. That requires a debt collector to not contact you again. Any further contact is a violation of the FDCPA. Or, you may have them prove you owe the debt by sending a validation letter certified return/receive requested to the debt collector. They are not allowed to contact you again in that example either, until they prove you owe the debt.