Unauthorized subscriptions

For the past 2 weeks I’ve got 9 messages: Y'ello. Thank you for subscribing to Reycreo_monthly at R9.00/mth. To cancel dial *155#. Call 135 for help. Ts&Cs www.mtn.co.za

About half an hour later I get the following message: Y'ello! You have earned FREE plays from MTN Cake Crush. Your plays will expire 23:59. Play to WIN. Dial *156*083# or www.mtnwin.co.za Ts&Cs

When I dail *155# there is always other subscriptions which I did not subscribed too.

I phoned the call center TWICE and still got another sms this morning.

Please advise how to block this from happening cause looks like your call center is incapable of doing their job and it’s costing me a lot of money!!!!