Third Party charged my account without my knowledg

l bought a phone on contract from MTN South Africa in September 2013 and things were fine in the last 23 months.l am only left with one payment. If l do not finish my airtime the maximum that l pay is R250 which is for the airtime that l use. Last month l did not use all my airtime and data bundles. What shocked me is when l checked the bill it was R683. I did not get the logic because l had not finished my airtime for last month. When l called the call center l was told that l had subscribed to an adverti*****t which was charging my account that l have with MTN. The money was not being deducted from my airtime or data bundles but it was being charged on my account which l have with MTN. How can MTN allow a third party to charge my account without my knowledge. I was even surprised when the call center told me that they are aware of those internet ****s. I was in the process of upgrading since l had finished my payments but when l saw the bill l cancelled the transaction and went back to prepaid.Can MTN please look into this matter because it will create a lot of legal fights which are not necessary.l dont know how many people have gone through the same trauma like mine.