About to be blacklisted for a account thats already been paid!!!!

I'm about to be blacklisted by lancet for a account that's already been paid close on a year ago. Their account department is a joke! We have sent them three times now our proof of payment to no avail or reply from them. Furthermore they keep harassing my mother who was listed as our emergency contact. Again despite us giving them the correct number three times they still keep phoning the wrong person. Absolutely shocking!!