Have a buddy system for dealing with angry residents so that the manager or employee is not alone or they have someone to call to assist them when/if an angry resident confronts them. Just having another person in the room may help diffuse the situation.
Have a separate room with a lock on the door and a phone for the manager or employee to go to if needed.
Ensure the employee knows when it is appropriate to contact the police for help. Emergency situations must be addressed promptly.
Have signals or code words to alert other employees when a manager or employee needs help.