MTN subscription management - unjustifed charges

My mother has a pre-paid MTN Number. She regularly gets SMS messages from MTN advising that she has successfully joined various subscriptions (charged at a daily rate - generally at R5/day). It appears that this is an opt-out option rather than opt-in. Is this legal? She has lost hundreds of rands off her pre-paid amount, unknowingly paying for these subscriptions. She is 93 years old and only manages to read SMS messages and take calls. There is no way that she would know how to opt in or out!! How do I stop this MTN? I keep having to check her phone and terminating these subscriptions. This is totally unacceptable!