If I am losing customers?

because one of my employees is proselytizing and harassing customers can I fire him? This is a sevice company and when he went to the customers home he argued with the customer and tried to force the customer to pray with him. The customer would not and threw him out and then he screamed out side of her house. The customer called me and said she does not need our services any longer. This is not the first complaint. He has been told about this twice before. He told me he will sue me if I fire him.