DSTV Lies about 4th month subscrition fee being free

I got an email in January stating that, if I paid on time for 3 consecutive months and I could enjoy entertainment on the 4th month at no extra cost. I did pay my account as early as possible for those 3 month and as promised had a credit loaded on my account for my April subscription. I was not aware of this credit been loaded on my account in April and went ahead and paid my monthly instalment. Once i discovered the credit, I did not request my payment back and opted to use it against my May subscription fee. On the 21st of May beknowing to me they reversed the credit discount given to me in April. I have been calling the call centre since 28 May to ask them to rectify but they can not seem to give me my credit back and they disconnected my subscription today and requesting me to pay my full amount and reconnection fee. So in other words DSTV are ***** and they have *********** staff in their call centre. I am seriously thinking of throwing my decoder away and moving to another option which may be cheaper anyway.