I had something similar happen except I really did have a loan which was repaid.
Nothing would stop them. Certified letters, numerous phone calls and it just kept coming. After nearly 10 years they turned it over to a collection agency and believe it or not, they discovered that I had already paid.
If a collection agency is more understanding than you then you are pretty bad.
Collection agencies are restrained by law and can be sued. The Feds aren’t and can’t be.