If you thought someone owed you money, would you stop calling?
A debt collector is not about to go away quietly. To a ''not me'' case I always recommend putting the agency on notice, just as you would if you were the target and the debt claim was invalid. Send a ''cease-communication'' notice via USPS Certified with return card. This is the method promoted by the FTC and many consumer watchdogs, and the only way to set a legal landmine, as such a notice is ignored at their risk of a lawsuit from you. You may already be due some payback if you can prove you've endured abusive or misleading treatment in an effort to collect debt.
Learn how to exercise your FDCPA rights as an alleged debtor at FTC-dot-gov. See also if your state laws are stronger.