Php Polling / blocking username instead of IP

[mage1]

how can i block the username of the person who already voted? heres my code

 

<?php
include("mysql.php");
$username=$_SESSION["username"];
mysql_query("select * from regusers where username='$username'");

echo "<strong>You already voted!<br /></strong>";

//connect to MySQL  
$db=new mysqli('localhost','root','');

if(!$db->select_db('poll1'))
    //create the database
    $db->query('create database poll1');
    
//select the database
$db->select_db('poll1');
        
//create table
if(!$db->query('select * from polldb'))
{    
    
    $db->query('create table polldb (id int auto_increment primary key, option1 int, option2 int, option3 int)');
   
    $db->query("insert into polldb (option1, option2, option3) values (0,0,0)");
}

//someone will vote
if($_GET['action']=='Vote')
{
    $option1=$_GET['option1'];
    $option2=$_GET['option2'];
    $option3=$_GET['option3'];
   
    
    
    if($option1!='' || $option2!='' || $option3!='')
    {
        //fetch initial values of polls in DB
        $result=$db->query("select * from polldb");

        $result=$result->fetch_row();
        $op1=$result[1];
        $op2=$result[2];
        $op3=$result[3];
    
        
        //increment the voted option
        if($option1=='voted') $op1++;
        elseif($option2=='voted') $op2++;
        elseif($option3=='voted') $op3++;
        
        //save the updated values
        $db->query("update polldb set option1='$op1', option2='$op2', option3='$op3'");
        
        //redirect to results page
        //by setting the $_GET var
        $_GET['action']='Results';
    }
}
if($_GET['action']=='Results')
{
    //fetch initial values of polls in DB
    $result=$db->query("select * from polldb");

    $result=$result->fetch_row();
    $op1=$result[1];
    $op2=$result[2];
    $op3=$result[3];
        
    //close DB
    $db->close();


?>