How to secure or disable the RSS feeds?

As pointed out in the comments by @kaiser, your question is very similar to this question. In fact, the question itself holds the answer. To disable all feeds add the following code…

function itsme_disable_feed() {
    wp_die( __( 'No feed available, please visit the <a href="'. esc_url( home_url( "https://wordpress.stackexchange.com/" ) ) .'">homepage</a>!' ) );
}

add_action('do_feed', 'itsme_disable_feed', 1);
add_action('do_feed_rdf', 'itsme_disable_feed', 1);
add_action('do_feed_rss', 'itsme_disable_feed', 1);
add_action('do_feed_rss2', 'itsme_disable_feed', 1);
add_action('do_feed_atom', 'itsme_disable_feed', 1);
add_action('do_feed_rss2_comments', 'itsme_disable_feed', 1);
add_action('do_feed_atom_comments', 'itsme_disable_feed', 1);

…to an empty plugin, and activate that plugin. This should prevent anyone from accessing a feed action.

Note that the above is sample code only, preferably you’d not wp_die but redirect or use a 404 instead.