When you spoke to someone at the collection agency, did you write down the account number, ss#, and dob of the guilty party? Did you speak to a manager at the agency? Did you call Sears directly and speak to a manager in their credit department? Did you get the actual address of the guilty party before it was changed to your dad's?

Be sure and document dates, times, and names of people to whom you spoke. The advice that others gave here is good to follow. There appears to be no easy resolution, as the guilty party has the same name as your dad.

(The problem I was having was that I "inherited" the collection agencies after I moved and got a new phone number. Evidently the people who had the number before me were owing people all over the place. Fortunately, I was able to stop most of them (the "live voices," anyway) by explaining that I just got the number, who I was, and writing down the information about the people they were looking for.)

One thing I do remember these agencies telling me, though, was that if I was asking them to not call me anymore, they would remove my number from their records. Have you tried telling the collection agency to not call you anymore? (It probably won't resolve the issue of your dad's credit report possibly being affected, but it MIGHT get them to stop calling him !!!.)

Hope all ends well.