Whitelist the usernames that can login to a ClassicPress site.

functions.php

function codepotent_disable_rogue_logins($user, $username, $pass) {

	// An array of usernames allowed to login.
	$allowed_usernames = [
		'coolChick582',
		'geekdude1',
		'some_other_username'
	];
	
	// Not an allowed username? Fail the login.
	if (!in_array($username, $allowed_usernames, true)) {
		remove_filter('authenticate', 'wp_authenticate_username_password', 20, 3);
		return null;
	}

	// Login ok to proceed.
	return $user;

}
add_filter('authenticate', 'codepotent_disable_rogue_logins', 10, 3);

Nice one! Thanks for sharing.