Security Finance Loan paid off months ago - company shows up at my door about once every three months Hudson, Wisconsin

This company apparently purchased a debt we had with our local hospital.  The account was never behind so why Security Finance purchased the debt in the first place is beyond me.  The company made several calls to our house, showed up at our doorstep and threatened to sue us if the account wasn't paid in full.  Over the course of several months the account was paid in full (after we argued with them over their interest rates and had it freezed).  Now - nearly two years later - they show up at my doorstep again saying we still owe them $110.25.  Here's the thing - a note was tucked into my door jam with the initals SBR and the amount owed.  When I call the company the person who answers says they have no idea what I'm talking about and to call back when the manager is around.  They were unable to identify who SBR was so now I'm wondering if a former employee is trying to scam us.  The other thing is this company said we owed more than we actually did the entire time they had our account.  We keep very detailed financial records and were able to prove exactly when and how much we paid.  Security finance said we owed nearly $1000 more than we actually did.  They can never seem to keep anything straight in their office and can't even identify the initials of someone who claims to work in their office.