MTN bad service

My son signed a contract at MTN Bethal for a R200 data package special. They then took R3000 from his account he went in and the lady said they made a mistake and didn't put it on the special. He must reverse the debit order. They will fix it. It happend 2 more times. Each time we went in with no assistance. Phoned head office. Still no assistance. He pays the R200 every month but the sim doesn't work. This is now from Feb 2020. Now he receives a sms saying he has been black listed!! Who can assist as MTN is useless!!