Account Settled and still being harassed for money

On 1 June I engaged with a consultant (Angel) to confirm outstanding balance due to settle account. She provided the info and I settled the account and emailed POP and requested settlement letter. Nothing was received to acknowledge or confirm. I continued to receive sms's notifications about payments due and emails about payments not received (with erroneous amounts reflecting in terms of what was required to be paid - way in excess of what was ever originally owed!). I emailed continuously and called multiple times...everybody was going to escalate my query and I would receive feedback - it never happened. I eventually got hold of Olivia Hoosain (Customer Care) who assured me she would escalate (17 July) - a month later - nothing but still getting SMS and emails of demand. I was told Sizwe (Manager would call me - he never did!) I called again and Olivia assured me he would. An agent called me... apparently Angel made an error with my calc and I owe money still... (no apology, no statement to prove this- I must just pay! Please can somebody senior call me to discuss - you cant mess with people like this and then not be willing to engage at an appropriate level? HELP