Refuse to cancel subscription

I made the mistake of taking the payment plan option with DSTV. The 24 month period runs out at the end of this month. I phoned in to cancel the subscription at the end of the 24 months last month. The agent told me this is impossible, and that I had to phone back this month. I phoned in on Monday again. I was told that the person cannot help me, and the the "Payment Plan" department will contact me in 24 hours. It is now 48 hours later and no one has contacted me. I do not know how it can be this difficult to cancel a fixed term agreement with a supplier. My overall experience with DSTV is very negative, as during last year they forced me to switch from debit order ot manual payments because they insisted on debiting more than I owed, one month they debited more than 2 month's subscription at the same time. This was fixed every time with a lot of hours waiting on the phone from me. Luckily I am on manual payment, so I will simply stop paying at the end of the 24 months