Retension not doing their Job right

I called early in the month of April to cancel my contracts which I knew were ending in May I even got two reference numbers but now I am being billed a 99.00 for 10gigs on both I mentioned very well that I dont wish to take any sim card only deals but they loading it into my account, please reverse this is not what I signed up for! I dont understands whats difficult to cancel a contract when its paid up!!!!!!!!!!