MTN MOMO spam messages have no option to OPT OUT

I keep getting MTN MOMO spam messages. I do not want the service. But MTN keeps sending me messages. The SMS says dial *135# to opt out but that thing does not have the option to opt out. I tried googling other ways to opt out but I am stuck. I get anything from 3-5 messages EVERYDAY!!! It feels like I am being punished for having a contract with MTN.