You deprive your six-year-old of dessert each time he fails to eat his spinach. In this sort of _______,

you weaken a response through taking away something pleasant or desire

feedback, reinforcement, punishment?

Definitely not reinforcement, but could be feedback or negative punishment. Reinforcement increases the response rate or amplitude.

Negative punishment

In this sort of situation, you are applying a form of punishment known as negative punishment. Negative punishment involves removing a desirable or pleasant stimulus (like dessert) in response to a behavior (failing to eat spinach) in order to decrease the likelihood of that behavior occurring again in the future. By utilizing negative punishment, you are aiming to weaken the response of your six-year-old not eating their spinach.