Impossible to cancel an MTN contract

MTN's service is absolutely shocking! I have been trying to cancel a contract since the 15th of April, to no avail. It is impossible to get through to the retentions department - 5 times I have had to hold for 45 minutes, after which I have given up. Who has 45 minutes to spend hanging on a phone during working hours? Their Facebook page has not been much help either. It seems I cannot cancel my contract, but MTN is quite happy to take my money every month for a service I no longer require. This is ****************!!!! The fact that their call load is "so high" is not really my problem.