PeterH:
You seem to be focusing on disabling servos (or joints) rather than making them work. It seems to me that making them work is the problem. I can't imagine why you would want to stop it working, but if so just disconnect/switch off/disable whatever part you want to stop working.To make it work you need to have a signal generator capable of controlling the position of as many servos as you need in your solution. Arduino could be that signal generator if you like. You would need some controller to define the movements that you wanted to achieve i.e. how the servo positions change over time. There are lots of different ways to achieve that, some that can be done on the Arduino. Have you decided what you want the robotic hand to do?
I'm focused on disabling joints, because I want to see if less joints are activated (if we had less joints in our fingers) how much movement would be restrained. Getting it to work will be a challenge. I just want the fingers to move on the hand, the wrist and other parts don't matter as that's not what I'm testing.