Hide posts from a certain category in the main query.

change_main_query.php

<?php
/**
 * Change the main query in order to hide posts from certain categories.
 *
 * @see https://codex.wordpress.org/Plugin_API/Action_Reference/pre_get_posts
 */
function my_customized_query( $query ) {
	if ( $query->is_home() && $query->is_main_query() ) {
		$query->set( 'cat', '-10,-11,-325' );
	}
}
add_action( 'pre_get_posts', 'my_customized_query' );