<?php

add_action( 'pre_get_posts', 'exclude_author_from_homepage' );

function exclude_author_from_homepage( $query ) {
    global $wp_the_query; // Stores a copy of the main query

    if (
        $query === $wp_the_query // Is this the main query we're on?
        && $query->is_home() // Homepage only, not single posts, etc.
    ) {
        $query->set( 'author', -123 ); // Exclude author ID 123
    }

    // No need to return anything because this is an action not a filter.
    // The variable passed to this function is done so via a reference.
}

?>