MTN spam messages

I have now tried for 6 months to stop these irritating pop-up messages from displaying on my phone. Sometimes MTN offers bundles at extra charges monthly and I fear my phone might have selected the go-ahead accidentally whilst in my pocket. This is very underhanded of MTN and no sms message to MTN *135# seems to stip this spam from coming through. Come on MTN, you should be a better company than this. Trying to con your customers into ***** offers... quite pathetic and highly irritating. Grow up MTN