In "The Changeling," why didn't Captain Kirk, as the "creator," simply tell Nomad that it was being reassigned? Tell the probe that its new job was going to be counting space particles (outside of the ship, of course)? And that he, assuredly, would check up on it after a set amount of time?
It doesn't seem like Nomad would question such a reassignment. Not at the beginning of the adventure, at least.
I'm looking for in-universe reasons.
Consider: he has a device, possibly a sentient AI, with the power of a starship, capable of firing effectively ON a starship, inside HIS starship, screwing with his crew and his ship, fully shielded and unable to be affected by any handheld weapons. Certainly enough to distract even a fine commanding officer like Kirk.
Nomad managed in less than 20 minutes to mindwipe one crewmember (Uhura) and kill the Chief Engineer, Mr. Scott. With the shock of Nomad being able to return Scotty to life, Kirk was pretty distracted and probably not at the top of his game.
By the time Kirk realizes what Nomad is, (39:40) he has accidentally revealed he is a biological unit and Nomad was no longer interested in following Kirk's instructions.
Nomad was also insane, possibly from the loneliness of deep space, possibly from the interaction with the alien probe, it is unclear whether Nomad would have accepted any instruction counter to its normal behaviors of sterilizing any life form that is unstable or in error.
This insanity allows Kirk to destabilize Nomad and send it into a self-destructive loop with the false and incorrect knowledge of Kirk's identity as Jackson Roykirk.
From Memory Alpha: Originally, Nomad was built by Jackson Roykirk for a deep-space extraterrestrial contact mission in 2002, but was lost in a meteor shower. During that time, it came into contact with Tan Ru, an alien probe whose mission was to collect and sterilize soil samples, presumably as a prelude to colonization. The two melded into this new Nomad, combining their technology and missions into one extremely powerful probe bent on sterilizing any imperfect lifeforms it encounters, using its own perceptions of perfection (i.e. itself) as a measuring stick. As such, it destroyed all life in the Malurian star system but because its original programming was damaged and corrupted in the merger, it erroneously equates Kirk with its creator, not realizing that the two are different people and its creator is, in fact, long dead.
