Stop the unending alerts MTN, they’re irritating!

I am sick and tired of being harassed by MTN. Literally, every hour I get bombarded with SMSes about how I can increase my airtime or load more data. Just today alone I have received over 20 SMSes. I called one of their service champions earlier to opt out of these alerts; she instructed me on how to deactivate the setting, but nothing has changed. These alerts are irritating me, and if there’s no way out then I’ll move to another network.