JQuery :: Validation - Ajax Submit With Captcha?

I use ajax submit for my jquery validation form. I NEED a captcha for the form, being I am getting attacked by bots. I cannot figure out how to do it. I will past my code below.

$('#contact_form2').validate({
submitHandler: function(form) {
var str = $("form").serialize();
$.ajax({
type: "POST",
url: "contactpage.php",
data: str,
success: function(){
$('#contact_form2').html("<div id='message'></div>");
$('#message').html("<h2>Contact Form Submitted!</h2>")
.append("<p>We will be in touch soon.</p>")
.hide()
.fadeIn(1500, function() {
$('#message').append("<img id='checkmark' src='images/check.png' />");
});}});
return false;
}