Disable RSS feeds in WordPress. Source: http://www.wpbeginner.com/wp-tutorials/how-to-disable-rss-feeds-in-wordpress/

disable-rss-feeds-wordpress.php

<?php

//* Disable RSS feeds in WordPress
add_action('do_feed', 'customprefix_disable_rss_feeds', 1);
add_action('do_feed_rdf', 'customprefix_disable_rss_feeds', 1);
add_action('do_feed_rss', 'customprefix_disable_rss_feeds', 1);
add_action('do_feed_rss2', 'customprefix_disable_rss_feeds', 1);
add_action('do_feed_atom', 'customprefix_disable_rss_feeds', 1);
add_action('do_feed_rss2_comments', 'customprefix_disable_rss_feeds', 1);
add_action('do_feed_atom_comments', 'customprefix_disable_rss_feeds', 1);
function customprefix_disable_rss_feeds() {
    wp_die(__('Sorry, but syndication has been disabled on this site. To be notified of a new post, please follow us on <a href="https://twitter.com/account-name/" target="_blank" rel="nofollow">Twitter</a>.<br/><br/>Back to <a href="' . get_bloginfo('url') . '">homepage</a>.'));
}