How to get rid from spam MySQL insert?

I really wonder that how someone adding records to my existing blog comment table. I am using my own custom blog script where user add comment on any particular blog. I have a separate comment table with blog id. I have tried several CAPTCHA technique but nothing works. I am using PDO for mysql. I am also doing JavaScript validation before submitting the form. I am not sure if he/she is real people or some bots are doing that. Within 1 hour thousands of records being added.

I am posing my code below. Can someone help me please?

<?php
if(isset($_POST['user_comment']))
{

if ($_SESSION['answer'] == $_POST['answer'] ) 
{
$name = $_POST['name'];
$email = $_POST['email'];
$website = $_POST['website'];
$web = $website;
$comment = addslashes(nl2br($_POST['comment_text']));
$comment =strip_tags($comment);
$id = $article_id;
$ref = $_SERVER["HTTP_REFERER"];
$userip = $_SERVER['REMOTE_ADDR'];
$useragent = $_SERVER['HTTP_USER_AGENT'];
$submit_date = date('Y-m-d H:i:s');

if ((($name) && ($email) && ($comment))) {

$conn = new PDO("mysql:host=$hostname;dbname=$database",$username,$password);


$sql = "INSERT INTO blog_comments (user,email,website,message,date,storyid,userip,useragent,block) VALUES (:user,:email,:website,:message,:date,:storyid,:userip,:useragent,:block)";
$q = $conn->prepare($sql);
$q->execute(array(':user'=>$name, ':email'=>$email, ':website'=>$website, ':message'=>$title, ':date'=>$submit_date, ':storyid'=>$id, ':userip'=>$userip, ':useragent'=>$useragent,':block'=>'1'));

        }

}
else
{
?>
<tr><td>
<p style="text-align:center; color:#CC0000; font-size:14px; font-weight:bold; padding-bottom:10px;">Wrong Answer! Please try again!!</p>
</td>
</tr>           

<?php
}
}
?>

THIS IS MY CAPTCHA CODE:

<?php

    session_start();

    $digit1 = mt_rand(1,20);
    $digit2 = mt_rand(1,20);
    if( mt_rand(0,1) === 1 ) {
            $math = "$digit1 + $digit2";
            $_SESSION['answer'] = $digit1 + $digit2;
    } else {
            $math = "$digit1 - $digit2";
            $_SESSION['answer'] = $digit1 - $digit2;
    }

    ?>  

THIS IS JS VALIDATION:

<script type="text/javascript" language="javascript">
function validate()
{
        if( document.getElementById( "name" ).value == ""){
           alert("Please enter your name.");
           document.getElementById( "name" ).focus();
           return false;
        }
        if( document.getElementById( "email" ).value == ""){
           alert("Please enter Your email ID.");
           document.getElementById( "email" ).focus();
           return false;
        }
        if( document.getElementById( "comment_text" ).value == ""){
           alert("Please enter your comment.");
           document.getElementById( "comment_text" ).focus();
           return false;
        }

        if( document.getElementById( "answer" ).value == ""){
           alert("Please solve this math.");
           document.getElementById("answer").focus();
           return false;
        }
}
</script>

THIS IS MY FORM:

<form action="" method="POST" onSubmit="return validate();">
<table cellpadding="5" cellspacing="5" width="100%">
<tr><td colspan="3">
<input type="hidden" name="id" value="<? print($id);?>">
</td></tr>

<tr>
<td style="font-size:12px; font-weight:bold; width:150px;">Your Name</td>
<td style="width:260px;"><input type="text" name="name" id="name" style="width:250px; height:20px;"></td>
<td style="text-align:left;">Required</td>
</tr>


<tr>
<td style="font-size:12px; font-weight:bold; width:150px;">Your E-mail</td>
<td style="width:260px;"><input type="text" name="email" id="email" style="width:250px; height:20px;"></td>
<td style="text-align:left;">Required</td>
</tr>


<tr>
<td style="font-size:12px; font-weight:bold; width:150px;">Your Website</td>
<td style="width:260px;"><input type="text" name="website" style="width:250px; height:20px;"></td>
<td style="text-align:left;">Optional</td>
</tr>

<tr>
<td style="font-size:12px; font-weight:bold; width:150px;">Your Comment</td>
<td style="width:260px;"><textarea name="comment_text" id="comment_text" style="width:250px; height:100px;"></textarea></td>
<td style="text-align:left;">Required</td>
</tr>

<tr>
<td style="font-size:12px; font-weight:bold; width:150px;">What's <?php echo $math; ?> = </td>
<td style="width:260px;"><input type="text" name="answer" id="answer" style="width:250px; height:20px;"></td>
<td style="text-align:left;">Required</td>
</tr>


<tr>
<td colspan="3" style="text-align:center;">
<input type="submit" class="submit" value="Submit" name="user_comment">
</td></tr>


</table>

</form>