http://www.ramonfincken.com/permalink/topic155.html

<?php
/**
 * Created on 21-jun-2009 11:45:26
 *
 * Phpbb3 codesnipplet -- hook_user_delete.php
 * @author  Ramon Fincken, RamonFincken.com WebsiteFreelancers.nl
 */

/**
 * Phpbb3 addon ( hook ) to initiate actions when deleting a user from the ACP using the standard user_delete() function
 *
 * @author  Ramon Fincken, RamonFincken.com WebsiteFreelancers.nl
 */
function my_user_delete()
{
   global $cache, $config, $db, $user, $auth, $phpbb_hook; // Note: Not sure if you need phpbb_hook, probably not :)
   global $phpbb_root_path, $phpEx;

   // User to delete: $user_id
   $user_id = intval($_POST['u']); // Note: it would be nicer to have this as a param, but this works just fine

   if($user_id > 0)
   {
      // Note: in my case I want to do some deletes, but feel free to
      // * call a function ( make sure phpbb has ACCESS to that function ! )
      // * do an insert in a "users_deleted" table, which can be read by your own code/CMS to perform actions later :)

      $sql = array();
      $sql[] = 'DELETE FROM some_table WHERE user_id = '.$user_id;
      $sql[] = 'UPDATE LOW_PRIORITY suggestions SET submitter = 0 WHERE submitter = '.$user_id;

      foreach($sql AS $key => $sql_temp)
      {
         $db->sql_query($sql_temp);
      }
   }
   return;
}


$phpbb_hook->add_hook('user_delete'); // This is a non standard hook, add this one without having to edit the core hooks file
$phpbb_hook->register('user_delete', 'my_user_delete', 'first'); // Tell hook to perform my_user_delete directly when the function user_delete is called
?>