Standing up to a bully can be as simple as saying “not funny” in a flat voice and walking away.

Standing up to someone does not require a speech or a clever comeback, and reaching for either usually escalates things. Flat and brief works because it refuses the game instead of trying to play it better. There is no heat to feed on and no audience moment to win. Walking off afterward removes the stage altogether.

Pick the line and practise it before you need it, because nobody invents one under pressure. Say it at normal volume, then go somewhere else. If it keeps happening, tell an adult who can actually act.