Unsolicited USSD spam from MTN

MTN keep sending me spam. I have blocked all their SMSs, but now they have started sending unsolicited USSD messages that I can not opt out of. I am also not finding a way to block them.

All the USSD messages are selling some unwanted service that will bill my account balance. I DO NOT WANT ANY SPAM SERVICES FROM MTN. I DO NOT WANT TO RECEIVE UNSOLICITED SPAM FROM MTN.

I have recieved messages for grocery deals and callerxchange / messagexpress thus far