<?php

add_action( 'wp', 'remove_comments_feed_for_posts' );
function remove_comments_feed_for_posts() {
    if ( is_singular( 'post' ) ) {
        remove_action('wp_head', 'feed_links_extra', 3 );
    }
}

add_action( 'template_redirect', 'disable_comment_feed_for_posts' );
function disable_comment_feed_for_posts() {
    if ( is_comment_feed () && get_post_type() === 'post' ) {
        global $wp_query;

        // Mark the current query as a 404
        $wp_query->set_404();

        // Return 404 HTTP status code instead of the default 200
        status_header(404);

        // By default, this page returns XML, so we change the Content-Type header
        // Because we want to show a 404 page
        header('Content-Type: text/html; charset=utf-8');

        // Render the 404 template
        get_template_part( 404 );

        // You should exit from the script after that
        exit();
    }
}