Nouns such as robot, machine, and automaton could work, though I think I have seen adjectives used in this situation more commonly.
People like that can often described as cold, sometimes frosty or icy. For example:
Don't go looking to Bob for sympathy, he's just cold.
Steve's frosty persona allowed him to sit through the saddest movies with completely dry eyes.
There's also the female-specific term Ice Queen:
I know you think you got that answer right, but there's no point in asking Mrs. Tinklebottom to change your mark; she's a total Ice Queen.
Oddly enough, I don't think I've ever heard a male-specific equivalent, such as Ice King, although I'm not sure why. I supposes you could try to use snowman as the male-specific equivalent, but people might think of Frosty the Snowman who was not unfeeling or heartless.