Contract misery

How do one get to cancel contract with MTN, I have 2 contracts with MTN, one ended in February, in March I have requested my number to be moved to prepaid and I was told It'll take 30 days. It's now June my number is still not moved to prepaid and I'm getting billed every month, my bill went up from R378 to R678. Call center sends me from pillar to post, I sent emails to MTN customer care no one is responding. Please help me get out of this misery