bad billing and accounts

After trying to cancel my account multiple times zone fitness kept billing me! I then took it into my own hands and reversed 2 months money they kept deducting. Which was R432. Now they have handed me over with a bill of R790, that will keep on increasing! I asked the creditors how they got to this amount. Their response to me is they don't know how. Zone just gives them amounts and they must get the money? Should they not have details? Zone fitness you leeches! I don't owe you any money!