Help! My Boss Is An Abusive Jerk

What action would you suggest staff take when the Executive Director of an organization is abusive to staff? Staff does not have access to HR. They report to the ED who reports to the board. Some incidents have been "investigated" by one or two board members and the HR of the employer of a board member. Nothing has improved. Incidents have been the ED slapping the hand, kicking, and yelling at an employee to "go do your f****** job," commenting on how an employee is dressed, yelling at staff, ED "forgetting" they did or said something, not following policies and procedures consistently, speaking harshly as to show their superiority. We are at a loss as to what to do. We are not permitted to speak to any member of the board without the ED's consent. We are a small staff of six employees. Suggestions would be most welcome.