Prevent all users from accessing the native Login and Registration page.

block-login-registration-page.php

<?php

// Prevent users from accessing the native Login and Registration page

// NOTE: This will prevent *everyone* from logging in through the standard channel. 
// Even you.

add_action(  'login_init', function () {
    if( ! is_user_logged_in() ) {
        // redirects to homepage
        wp_redirect( '/' );
        exit;
    }
});