[PeaceCoder]

SELELECT id 
FROM users U 
WHERE 
  ISNULL((SELECT id 
    FROM banUsers 
    WHERE WhoBanID='{$userID}' && WhomBanID=U.id || WhomBanID='{$userID}' && WhoBanID=U.id
    ))

Структура banUsers

{
  id : int(11) auto_increment,
  WhoBanID: int(11) unsigned,
  WhomBanID: int(11) unsingned
  }