Too much premium for data i asked to never renew

I took a data contract with Cellc and it expired on the 28th of November,before that i received countless calls from their agent asking me if i would like to upgrade my data and i told all of them that i would not like to renew the contract and even asked them to make a note on their side but on the 1st of december i received an sms that says data was loaded on my sim card,i escalated the issue and they called me and told me that it was my fault i was supposed to call in and cancel the contract myself and i understood that but what ****ed me of is the amount of money they charged me right now,how can the same data i have been receiving move from R514 to R900 where do they expect i will get that kind of money when i only budgeted for my cellphone upgrade of R700 and now i must pay R1800 i never made budget for, I have been their customer forever but this kind of treatment will make them lose me as their customer because clearly they dont think for their customer,i know i was supposed to call and cancel but what is the purpose of their agent if am gonna tell them make a note i don't wana renew my contract and they still went ahead and sent me data, I dont have R1800 lying around its december and expenses are a lot already for me to be paying money i never budgeted for,am really ****ed i dont wana lie,cellc really did me dirty