Block user with a script

That is the correct way. At the same time, the recommended, in terms of compatibility with features releases of WP Cerber, way is:

include_once( 'set here your plugins directory path' . '/wp-cerber/cerber-load.php' );

if ( function_exists( 'cerber_load_admin_code' ) ) {
	cerber_load_admin_code();
	cerber_block_user( $user_id, $msg, $note );
}

/*

Parameters are:

$user_id = User ID
$msg = An optional message to be displayed on the login page when the user tries to log in
$note = An optional note visible to website admins

*/