FNB and Barrett & Assc Legal

I have tried the call centre, legal collections, get-back-to-me-FNB , now trying hello peter to get FNB to contact me please. I have a paid up/account closure letter from FNB dated 21/05/2018 , yet I still get harassment calls and spam sms from Barrett & Assc Legal claiming to act on behalf of FNB. They kindly offer a discounted settlement (would love to see a settlement offer on a zero balance??!!) and refuse to accept that I do not owe FNB anything . Since this collections office claim that FNB instructed them, could FNB kindly instruct them to BACK OFF my case as they are wasting my time. I would appreciate feedback and action on this matter.