cell c ripoff

i used my cell c data contract for two years paying abt R100 for 2 gig and the contract expired nov 2014, as i had no notification after the expiry date i continued using the sim only to realize i was paying about R380+ per month of data, i then enquired of cell c only to be told my contract had expired and was now being billed per usage. i continued using the line until i could not take on the high charges i then requested cell c to cancel the contract which i was told they did, now i go a call to be told im owing R525 for data which i never used and all im told is it was benefit sent to the sim, thisis just a complete ripoff because how can i be iven benefits on an expired contract, please help me understand i have been trying to cancel the contract but all cell c does is billing me with expiring data and huge amounts. Please cancel the expired contract which i dont see any reason to pay for unused data..*** this is unearthical to bill a terminated or even expired contract just a *******. please address this, i do not have an extra R500 to appetize cell c.