spoil (someone's) appetite

Translations
портить (чей-то) аппетит
If you eat a candy bar now, it will spoil your appetite for dinner.
Если ты съешь шоколадку сейчас, это испортит твой аппетит перед ужином.
Definitions
To make someone lose their desire to eat, often by giving them food or snacks before a meal or by exposing them to something unpleasant that affects their willingness to eat.
Words
appetite

noun
аппетит
spoil

verb
портить