Vodacom refuses to cancel contract

I was called by Valene from customer services from Mondeo in association with vodacom on the 18/03/2016to say that they are offering me less pricing rate on my data of R119 per month instead of R149 for my monthly data. I then asked her if i would have to pay 119 on top of what i was already paying for my contract and she say i will be paying 30 rand less than what i am paying currently. I then called the customer service number again within 10min after having agreed to take the offer and spoke to phindile who then said i was going to pay R119 more... I then asked them to cancel it and she said it couldnt be cancelled and i would have to return the offered goods (free wifi router and educational voucher) after i have recieved it..I feel taken advantage of by the initial sales lady.She was beeing dishonest and jus wanted me to agree to the deal. I feel i have a right to cancel the agreement before they deliver anything and make any deductions from my account because she **** and was beeing *********.