MTN & Saya Recoveries taking money from my account with a rejected mandate

Needed to make a payment arrangement towards my account, called MTN and upon talking to one of the agents I was told that another agent will call me back to further conclude said arrangement. No agent ever called me, until another agent called me from Saya Recoveries because I was supposedly handed over. I mentioned to the agent that I had a pending discussion with MTN before I can agree to a debit mandate on my account. When they did send the mandate which I rejected as clearly seen on my banking app. MTN still has not responded back to me about my arrangement with them, Saya recoveries continues to take money from my account even after I rejected the mandate pending my outstanding communication with MTN. When I tried seeking clarity from Saya Recoveries, I spoke to one Gugu Xulu, who could not offer any assistance, rude and pompous to no end.