********** vodacom wont cancel my contract

I took out a contract with Vodacom in the month of Nov for a 10g modem. Within 1 day of rwceiving the device i called vodacom as 3g had dissapeared in less than an hour and i sent back the device. They collected the device and said they will cancel the contract but until today they have done no such thing. Its been a long dec with me calling the useless call centre over 50 tmes but i keep getting threats that my account will be handed over and this **** vodacom thing is continuing to bill me. I have had enough and all theyvdo is send you from pillar to post.

Why cant they just cancel the contract as they keep promising. What on earth am i being charged for.