Disgusting service

I am so disgusted with MTN! I have been with MTN since 2012 and for the first time last year I downgraded my contract because I didn’t want to upgrade and every month my account gets suspended and when I call in they apologise and say that they have a new system! This is the 4th month that it’s happened. I call in today and they tell me I went over my spend limit, I DO NOT EVEN HAVE A SPEND LIMIT, I have never had a spent limit in my life. I even paid in advance just incase and this still happens! I have finally complained and no one has responded! What is the issue with MTN you *******!