WANT TO CANCEL CONTRACT

I received a statement where it state that i am behind with my account of R939.19 After numerous emails i have send to *** they still send month in and month out i did not pay. My contract is for R99.00 per month and they charge me R299.19 and then again R355.00 and other amounts ass well. I think they employ wrong people in positions that dont know how to do their work. I wont pay that amount because i dont owe them anything.