Annoying SMSs after recharging

Just about every day now I am receiving spam mail from MTN and it's highly annoying. \MTN Money saving Tip! Don't use airtime to surf the web! Recharge with a bank card & buy data bundles! Dial *141*10# ts&cs. Opt Out 30246"In my experience opting out of these things usually gets more spam delivered to your phone because you get added to more spam lists. Here is another: \""Thank you and congratulations! your number has been nominated for the free chance to win R40"