Again being charged more then stipulated on my con

Cant we just get rid off the individuals in Cell C that is constantly trying to steel the clients money, It took me 6 months in 2014 to sort out over billing and in the end I was only refunded half my money. A debit order went off this morning and I was charged R 80-00 more than the previous months This is contract breaking on behalf off Cell C. I signed 4 x 24 month contracts March 2014 and the total for the for contracts ( straight up top up 200 ) is R 996-00 per months and I don't see why they can just change a written agreement as they please. So I will then just go and cancel my debit order I will not pay them in full the end off March for April because Cell C is steeling from me, if they take R 80-00 from every second client they making a lot off money.