ABSA Harassing about already paid up amounts

I am so disappointed, shocked and emotionally tired. Long story short I went onto debt review willingly which I finished in March / Feb this year. Where we paid off all my debt and since I received my clearance certificate and paid-up letters in March 2019 I've been harassed by ABSA's debt collection agency on and off every few months saying that I still owe ABSA a huge amount, which can not be possible because according to the paid-up letter and clearance certificate that I have this account that they are now taking me to lawyers about was paid up last year already. Please this harassment needs to stop