MTN Spam - It Just Won't Stop!

I am sick and tired of MTN's constant bormbardment of my cellphone number with their marketing messages. Every day I receive a minimum of 2 messages and despite having opted out using the *135# USSD code as well as typing STOP as a reply, they persist in sending me these dumb messages.

Surely this is against the direct marketing laws in South Africa? What can be done about it?