Disable cache for a block in Drupal 8

Code snippet that can be used to disable caching for a block in Drupal 8.

use $vars['#cache']['max-age'] = 0 to stop  block being cache in drupal 8.

/**
 * Implements hook_preprocess_HOOK()
 */
function mytheme_preprocess_block(&$vars) {
  if($vars['derivative_plugin_id'] == 'block-id-name') {
    $vars['#cache']['max-age'] = 0;
  }
}

for custom Block you can use getCacheMaxAge() method like this:

class MyModuleBlock extends BlockBase {
  /**
   * {@inheritdoc}
   */
  public function build() {
  }

  /**
   * @return int
   */
  public function getCacheMaxAge() {
    return 0;
  }
}

Also you can use \Drupal::service('page_cache_kill_switch')->trigger(); to disable the cache for anonymous users and all others too.

Example:

function fest_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
     if ($form_id == "node_something_form") {
           	//disable the form cache for anonymous users and all others too
	\Drupal::service('page_cache_kill_switch')->trigger();
	}
}