In the movie "Ex Machina" we reveal that Nathan has in fact created multiple AI robots before Ava, many which had started to express hostility towards him. We would think that when creating Ava, he would include a "master override password" (not even getting into the 3 Laws of Robotics) of some sorts that when spoken, would completely disable the AI. It seems very unusual that Nathan would either overlook or intentionally exclude this safety percaution. Was this because he wanted to make Ava as realistic as possible or for some other reason?
It's been a while since I watched Ex Machina, but I remember Nathan's arrogance. He thought he was in control of everything. He thought he was several steps ahead of his 'bots and his human guest. He thought he had them trapped in the environment he had created. He also thought that since he had created his 'bots, he was their "master".
To his narcissistic mind, there was no need to put in a fail-safe like a master override password. (I don't have any outside sources to back up my conclusion, but it fits with Nathan's personality.)