/**
 * Implements hook_block_access().
 */
function {{ machine_name }}_block_access(\Drupal\block\Entity\Block $block, $operation, \Drupal\Core\Session\AccountInterface $account) {
  // Example code that would prevent displaying the 'Powered by Drupal' block in
  // a region different than the footer.
  if ($operation == 'view' && $block->getPluginId() == 'system_powered_by_block') {
    return AccessResult::forbiddenIf($block->getRegion() != 'footer')->addCacheableDependency($block);
  }

  // No opinion.
  return AccessResult::neutral();
}