FloodProtectedForm example of the Drupal flood service. Take from https://www.hashbangcode.com/article/drupal-9-integrating-flood-protection-forms

FloodProtectedForm.php

<?php

namespace Drupal\mymodule\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Drupal\Core\Flood\FloodInterface;
use Drupal\Component\Utility\Crypt;

class FloodProtectedForm extends FormBase
{

  /**
   * {@inheritdoc}
   */
  public function getFormId()
  {
    return 'mymodule_flood_protected_form';
  }

  /**
   * The flood service.
   *
   * @var \Drupal\Core\Flood\FloodInterface
   */
  protected $flood;

  /**
   * FloodProtectedForm constructor.
   *
   * @param \Drupal\Core\Flood\FloodInterface $flood
   *   The flood service.
   */
  public function __construct(FloodInterface $flood)
  {
    $this->flood = $flood;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container)
  {
    return new static(
      $container->get('flood')
    );
  }

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state)
  {
    $form['text'] = [
      '#type' => 'textfield',
      '#title' => 'Flood protected field',
      '#required' => TRUE,
    ];

    $form['submit'] = [
      '#type' => 'submit',
      '#value' => 'Test',
    ];

    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state)
  {
    $text = $form_state->getValue('text');
    $hashedValue = Crypt::hashBase64($text);
    $floodControlUserIdentifier = $this->getRequest()->getClientIP() . $hashedValue;

    if (!$this->flood->isAllowed('mymodule.flood_protected_form', 10, 3600, $floodControlUserIdentifier)) {
      $form_state->setErrorByName('url', $this->t('Too many uses of this form from your IP address. This IP address is temporarily blocked. Try again later.'));
      return;
    }
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state)
  {
    $text = $form_state->getValue('text');
    $hashedValue = Crypt::hashBase64($text);
    $floodControlUserIdentifier = $this->getRequest()->getClientIP() . $hashedValue;

    $this->flood->register('mymodule.flood_protected_form', 3600, $floodControlUserIdentifier);
  }

}