Struggling to cancel my contract yet I paid in full!!!!

I am desperately to cancel one of my contracts where they changed from data+minutes+smses to minutes and smses only. One of the call center agents said I must pay immediately which I did, then the second agent said she is cancelling but she put through the cancellation for a month later, meaning they want to bill me for another month! After I have cancelled! Needless to say, I went through 5 call center calls in order to cancel and up to now i don't have confirmation of immediate cancellation, even though I paid in full today. I have been a loyal customer for so many years and i am disgusted by them trying to cheat me into another month of paying for stupid smses!!!!!