Supersonic Bad Service

It's been two years and STILL Supersonic has FAILED to action a credit of R250 due to me! I've called and spoke to numerous people, I even went to their offices 14 Avenue and sat down with MTHETHO who acknowledged their mistake and promised to action this credit but in vain! Till today I still receive invoices for this amount!!! I'm exhausted with this company Please help me, what else can I do