Block user upon one webform submission
The best way to do this is to write your own webform handler and attach it to the webform's postSave() method. You can also experiment and try another method if needed from WebformHandlerBase.

Webform handlers are not documented well, but they should be because they are quite useful.

modules/custom/mymodule/src/Plugin/WebformHandler/MyModuleWebformHandler.php

&lt;?php

namespace Drupal\mymodule\Plugin\WebformHandler;

use Drupal\webform\Plugin\WebformHandlerBase;
use Drupal\webform\WebformSubmissionInterface;

/**
 * Webform example handler.
 *
 * @WebformHandler(
 *   id = "mymodule_webform_handler",
 *   label = @Translation("My Module Webform Handler"),
 *   category = @Translation("My Module"),
 *   description = @Translation("Block users after submission"),
 *   cardinality = \Drupal\webform\Plugin\WebformHandlerInterface::CARDINALITY_UNLIMITED,
 *   results = \Drupal\webform\Plugin\WebformHandlerInterface::RESULTS_PROCESSED,
 *   submission = \Drupal\webform\Plugin\WebformHandlerInterface::SUBMISSION_OPTIONAL,
 * )
 */

class MyModuleWebformHandler extends WebformHandlerBase
{

  /**
   * {@inheritdoc}
   */
  public function postSave(WebformSubmissionInterface $webformSubmission, $update = TRUE)
  {
    if (!$webformSubmission-&gt;isDraft()) {
      $webformSubmission-&gt;getOwner()-&gt;block();
    }
  }

}


After getting that setup, enable your new module (don't forget an info.yml file) and then edit your webform settings > handlers to add this to that particular form.