Replace entity access handler to own

/**
* Implements hook_entity_type_alter().
*/
function my_module_entity_type_alter(array &$entity_types) {
 if (isset($entity_types['privatemsg_thread'])) {
   $entity_types['privatemsg_thread']->setAccessClass(\Drupal\my_module\Access\MyCustomAccessControlHandler::class);
 }
}