Not happy

I took a new contract yesterday that is was going to charge me R99 for 3 years however my debit check mandate it is stilling me about R198 which I can't afford, today when I call to cancel they are telling they can't assist me with cancelation how come cause it is not yet even 24hours.Can I cancel this contract cause they even telling me that 10GB is for promotion and what will happen to me when the promotion end and it is something I was not told yesterday