Exclude some category from post list

Try something link this:

function exclude_category( $query ) {
    if ( $query->is_posts_page && $query->is_main_query() ) {
        $query->set( 'cat', '-1 -3' ); // -{ID} categories to exclude comma separeted list
    }
}
add_action( 'pre_get_posts', 'exclude_category' );

Sorry my friend, where i put this code? In fuctions.php (child theme)?

Ok in fuctions.php thanks m8

Oh sorry giorjoe, I did not specify because I knew that you already knew it ??