ho provato a sistemarla

Codice PHP:
<?php      
session_cache_limiter('nocache');     
session_start();          
$user=$_POST['nome'];     
$pass=$_POST['password'];           
if(isset($user, $pass)) {              
include("connect.php");     
$conn=mysql_connect($HOST, $USER, $PASSWORD);     
$db_ok=mysql_select_db($DB, $conn);     
$sql="SELECT nickname,pwd,livello FROM utente WHERE nickname='$user' and pwd ='$pass'";     
$row=mysql_fetch_assoc($ris);        
$count=mysql_num_rows($ris);     
if($count==1){          
$livello=mysql_result($ris, 0, "livello");                  
if($row["livello"]=='Esterno'){     
$_SESSION["sess_var"] = "1";     
header("location:menu_livello_2.php");         }     
else{         
if($row["livello"]=='Interno'){         
$_SESSION["sess_var"] = "2";     
header("location:menu_livello_3.php");             }         
else{             
if($row["livello"]=='Amministratore'){             
$_SESSION["sess_var"] = "3"; 
header("location:menu_livello_4.php");             }             
else{              
header("location:menu.html");                  }             
}         
}          
 }     
 mysql_close($conn);     }               
else {    header("location:menu.html");     }                    
?>
il log-in la fa solo che ho ancora il problema di prima , ovvero tornando indietro da browser voglio che si perda completamente i diritti di log-in e non possa più tornare avanti.