lotus.module

function lotus_cron() {

  $result = \Drupal::entityQuery("user")
    ->condition('login', strtotime('-6 months'), '<=')
    ->execute();

  $storage_handler = \Drupal::entityTypeManager()->getStorage("user");

  foreach ($result AS $user) {
    $entity = $storage_handler->load($user);
    $entity->block();
    $storage_handler->save($entity);
  }

}

We can use \Drupal\user\Entity\User::load as well, its gonna be:

function lotus_cron() {

  $result = \Drupal::entityQuery("user")
    ->condition('login', strtotime('-6 months'), '<=')
    ->execute();

  foreach ($result AS $user) {
    $entity = \Drupal\user\Entity\User::load($user);
    $entity->block();
    $entity->save();
  }

}