MTN keeps spamming me with marketing via USSD push

Every day, several times a day, MTN pushes a USSD session to my phone which takes over the whole handset no matter what I am doing. The messages are stupid things like \CLICK ACCEPT FOR MORE WOW BONUS"which you can either accept or decline. You can't use your phone unless you do.<br> <br> This is an insidious marketing tactic and I cannot believe it is legal to send messages using USSD which literally hijack your phone until you accept or decline. You can't even opt-out. I will be complaining to WASPA but please everyone else who gets these stupid message"