Remove the WP REST API JSON Endpoints for everyone except Administrators

functions.php

<?php
/*
 * Only allow Admin users to view WP REST API JSON Endpoints
 */
function mytheme_only_allow_logged_in_rest_access( $access ) {
	if( ! is_user_logged_in() || ! current_user_can( 'manage_options' ) ) {
		return new WP_Error( 'rest_cannot_access', __( 'Only authenticated users can access the REST API.', 'disable-json-api' ), array( 'status' => rest_authorization_required_code() ) );
	}
	return $access;
}
add_filter( 'rest_authentication_errors', 'mytheme_only_allow_logged_in_rest_access' );