MTN suspending account and bad aftercare sales

Y’ello! Thanks for using MTN. Your request with ref no *** has been submitted. Get 1GB FREE monthly on Ayoba: https://i.ayo.ba/dQjW/iscc

I am receiving the above message on a daily basis from MTN on my cell phone. They also blocked my account now for how many times, I pay my monthly account each month but I have send them email on a regular basis asking them to please sort out my account, I also involved ICASA bit no one is coming back to me. Each month they are adding R301 to my account and my account is going into the arrears. I am a customer with MTN now for 15years and this is how they treat a person. I can't be without my cellphone as this is my work and landlines. Can someone please assist me, as I am totally stress up with all this threatening messages from MTN.