Telkom mobile *****ing money

I had a Samsung Galaxy tab on a 2 yr contract at R300 per month, when the contract came to an end I was called about an upgrade, I insisted that I did not want to upgrade but wanted it cancelled as the 2 years were up, Telkom refuses to do that and sent me an account this month for a whopping R986 , more than triple my originsl monthly payment, their attitude is that I will pay this and they will not stop taking any amount they wish, I suspect someone there helped themselves to a device that they now expect me to pay for, as soon as my contract stopped their service did the same