Prevent PHP form submit depending on Ajax form response

<form action'#' method='post' id='account-add'>";
Card No<br/>
<input type='text' name='cardno' id='cardno'/><div id='disp'></div>
<input type='hidden' name='postback' value='add'/>
<input type='submit' id='submit' value='Add' />
</form>
	
<!--jquery validation-->
<script src="https://cdn.jsdelivr.net/jquery.validation/1.15.0/jquery.validate.min.js?v=1"></script>
<script src="https://cdn.jsdelivr.net/jquery.validation/1.15.0/additional-methods.min.js?v=1"></script>
<script>
$().ready(function() {
					$.validator.addMethod("cardno", function(value, element) {
							return this.optional(element) || /^xyz[0|1|9]\d{5}X$|^LYBLA[2|3|4|9]\d{6}$|^TMP\d{9}$/i.test(value);
					}, "Card Number is invalid. It should be xyz, 6 digits ending with X, or 7 digits");
$("#account-add").validate({
		rules: {
			cardno: {cardno:true,
							required:true},
		},
		messages: {
			cardno: "Card no must be in the format xyz followed by 6 digits ending in X, or 7 digits.",
		}
	});

});
</script>

<!--Ajax call-->
<script type="text/javascript">
$(document).ready(function(){
$("#cardno").change(function() {
var cardno = $('#cardno').val();
if(cardno==="")
{
$("#disp").html("");
}
else
{
$.ajax({
type: "POST",
url: "match-cardno.php",
data: "cardno="+ cardno + "&companyid=xyz",
success: function(html){
$("#disp").html(html);
}
});
return false;
}
});
});
</script>

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<?php session_start();

include 'config.php';


if(isset($_POST['cardno'])){
$cardno=$_POST['cardno'];
$companyid=$_POST['companyid'];

//check card exists in list of possible cards
$sql="select cardno from tblcardpossibles where cardno =? and companyid=?";
$params=array($cardno, $companyid);
$options =  array( "Scrollable" => SQLSRV_CURSOR_KEYSET );
$result=sqlsrv_query($conn, $sql, $params, $options);
include "error.php";

$row_count = sqlsrv_num_rows($result);
if($row_count=='0'){
echo "<p class='error'>You have entered an invalid card number. Please check your entry. </p>";}


//Check if card has already been issued
$sql="select title, firstname, surname, postcode from tblcardsissued where cardno =? and companyid=?";
$params=array($cardno, $companyid);
$options =  array( "Scrollable" => SQLSRV_CURSOR_KEYSET );
$result=sqlsrv_query($conn, $sql, $params, $options);
include "error.php";

$row_count = sqlsrv_num_rows($result);
$row= sqlsrv_fetch_array( $result, SQLSRV_FETCH_ASSOC);

if($row_count<>0){
$title=$row['title'];
$firstname=$row['firstname'];
$surname=$row['surname'];
$postcode=$row['postcode'];
echo "<p class='error'>Card already issued to $title $firstname $surname, $postcode</p>";}
}

?>

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