African Bank... Help

I took out a loan at african bank a few years ago. Due to circumstances, i cannot have the debit order go off every month and have requested on more than one occasion that they switch it to manual payment from my side. Despite numerous requests and explaining my current situation they insist on debiting my account with strange values all thru the month, sending my account into negative. They also do this all the time, like last night an amount went off? This is unacceptable as i have explained and the people i spoke to said they would swtich it. This has been going on for over 6 months now? What can i do?