Markhams blocks cell phone

As an account holder with Markhams for many years, i was offered an airtime package of R55.00 airtime with 25 sms's for an amount of R40.00 per month, which i gladly accepted and have had for a number of years now. I received the airtime and sms's on the 6th of each month, and the amount for R40.00 was added to the payments i made at the end of each month. About 5 months ago, the 25 sms's as part of the package, was without notifying me, stopped. I called Markhams to query this and was told that the service provider (MTN) cancelled the 25 sms's, so in other words it is no longer part of the package. I asked why i wasn't informed of this change, the staff member simply said "sorry." Over the months that followed, the airtime i received each month was always less than the R55.00, so again i approached Markhams, this time to cancel this arrangement, which was around about the 8th August. I was told that i needed to give them a one month notice of cancellation, which i did. A few days later my cell phone was completely blocked, i could not make nor receive calls. Again i approached Markhams who, after a considerable amount of time, unblocked my cell phone. On the 6th September, i received my last installment of airtime. On Saturday the 8th September, my cell phone was again blocked, and has been ever since. Does Markhams have the legal right to block my cell phone simply because i have cancelled this agreement? And the cancellation period has not even finished! Am i going to be compensated for the inconvenience and the money spent for the airtime purchased for this month????