Pathetic Service

I thought making use of their app to make payments would be much quicker for them to receive it but I guess I was wrong, it's even more longer then an eft payment. I paid partially to my account on the 25th but the very next day they suspended my services. So I decided to pay the remaining outstanding balance to get my account up to date. All of a sudden they can't seem to find that payment, can't allocate it and has been harassing me ever since for the payment of R722 after sending them 8 different forms of payment notifications and proof that I've paid my account in full. I've used every email address there possibly is of MTN to get the message out to them even had to email their consultants directly with whom I've spoken to on their 135 number. I went into their local branches as well and that consultant also send them all the proof through but they still keep sending me emails requesting me to make a double payment. ***s can start to fly, violets will begin to turn red and still I won't pay them what they want from me as I did pay my accounts in full with them and all the proof is there. Where can I open a lawsuit against them please as this is getting too much, too much for me now having to deal with these threatening and harassing emails everyday.