Can I refuse to testify at an arbitration hearing if I no longer work for the company,they are going to supbonea me?

Unless and until you are served with a subpoena, you are not obligated to appear and testify. However, once you have been served with a properly issued subpoena (every state has different rules for issuing subpoenas), you are obligated to appear and can be sanctioned if you chose not to do so.