WordPress Block Spam Comments: no plugin, no dependencies, pure JavaScript, place in your functions.php

wpBlockSpamComments.php

<?php
/* Block Spam Comments - BEGIN */
/* source: http://davidwalsh.name/wordpress-comment-spam */
add_action( 'wp_footer', 'catch_new_comment_submit' );
add_filter( 'preprocess_comment', 'preprocess_new_comment' );
function catch_new_comment_submit(){
	global $post;
	if( comments_open( $post->ID ) && (is_single() || is_page()) ){ ?>
<script type="text/javascript">
window.onload = function() { 'use strict';
	var commentForm = document.getElementById('commentform');
	function serialize(obj, prefix) {
	  var str = [];
	  for(var p in obj){
	    if(obj.hasOwnProperty(p)){
	      var k = prefix ? prefix + '[' + p + ']' : p, v = obj[p];
	      str.push( typeof v == 'object' ? serialize(v, k) : encodeURIComponent(k) + '=' + encodeURIComponent(v) );
	    }
	  }
	  return str.join("&");
	}
	function processCommentForm(e) {
		if(e.preventDefault) e.preventDefault();
		commentForm.elements['submit'].disabled = 'disabled';
		commentForm.elements['submit'].style.visibility = 'hidden';
		var formData={};
		for(var i=0; i<commentForm.elements.length; i++)
			formData[commentForm.elements[i].name] = commentForm.elements[i].value;
		formData['is_legit'] = 1;
		formData = serialize( formData );
		var request = new XMLHttpRequest();
		request.open('POST', commentForm.action, true);
		request.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded; charset=UTF-8');
		request.send( formData );
		commentForm.elements['submit'].disabled = false;
		commentForm.elements['submit'].style.visibility = 'visible';
		location.reload();
		return false;
	}
	if(commentForm.length){
		if(commentForm.attachEvent){
			commentForm.attachEvent('submit', processCommentForm);
		} else {
			commentForm.addEventListener('submit', processCommentForm);
		}
	}
};
</script><?php 
	}
}
function preprocess_new_comment( $commentdata ){
	if( !isset($_POST['is_legit']) )
		wp_die();
	return $commentdata;
}
/* Block Spam Comments - END */