MTN refused to cancel contract after paying off

My MTN cellphone Contract ended in May after 24 months and I called MTN call center to cancel because it was payed off but they gave excuses so they can have enough to renew it without my concern. This followed after I received several phone calls and messages to upgrade my contract, which I refused. They emidiately sent airtime and sent an invoice with another opening and closing balance. This is unacceptable. I am tired of trying to get hold of this people because you spend atleast 20 minutes waiting on the line and they say it's normal.