Poor billing

I took out two Sim only contracts with mtn n I asked both to be kept so that I don't have extra spend when I finish my data was promised that it was kept bt to my surprise every month starting from December my bill keeps on going up every month. Went to mtn Carlton center n I waited for an hour only to be told that they can't do anything I must dial *155# n follow the prompts to stop extra usage n I Dd that but to date my bill is still high every month.

When I took this contracts they where Sim only contract and was told I can cancel any time so I would like to cancel all my contracts under my name with you n leave my Sim cards as prepaid.