Erroneous amounts being charged/debited

I am debited amounts I am not sure where they come from. My contract of which I've decided to cancel is only R100.00. For the past month I received a statement saying I owe R404.34 I have since settled the amounts as they appear on my statement and to my surprise for this month I received a statement saying I owe R307. I will not pay this amount because not even the statement makes sense at all. I need this contract to end ASAP as I am dissatisfied with the services received