1. Stop calling and emailing. More than once is too often.
2. "Justified" is far from relative. Either a chargeback is supported or it is not. I don't know the facts of your contract and what you refer to as "fine print" but if you agreed to something and then dispute it through a chargeback, that is not a justified chargeback and it would not be odd for the merchant vendor to no longer do business with you.

Whether Sixt has fired you because of the chargeback incident or something else, the fact is that it apparently no longer wants you as a customer. I would move on and use another agency.