Harassment by MBD Attornyes

For 6 years MBD had een harassing me for what they consider outstanding debt. I had an account with Cell C. I paid the account in full and migrated to another network. Cell C refused to allow the migration unless the full amount of the contract was settled which is what I did. Since then MBD had been harassing me for payment. Is it possible to lay a complaint for harassment against this company? How do I get them off my back?