Easily Prevent Comment Spam:

The problem.
Comment spam is such a pain for everyone. Akismet is a good solution, but why not block spammers outright instead of just marking their comments as suspected spam? This code looks for the HTTP referrer (the page where the request comes from) and automatically blocks the comment if the referrer is incorrect or not defined.

The solution.
Paste the following code in your functions.php file:

function check_referrer() {
    if (!isset($_SERVER['HTTP_REFERER']) || $_SERVER['HTTP_REFERER'] == “”) {
        wp_die( __('Please enable referrers in your browser, or, if you\'re a spammer, bugger off!') );
    }
}

add_action('check_comment_flood', 'check_referrer');

That’s all. Once you’ve saved the file, your blog will have a new level of protection against spam.

Code explanation.
This code automatically rejects any request for comment posting coming from a browser (or, more commonly, a bot) that has no referrer in the request. Checking is done with the PHP $_SERVER[] array. If the referrer is not defined or is incorrect, the wp_die function is called and the script stops its execution.

This function is hooked to WordPress’ check_comment_flood() function. This way, we can be sure that our check_referrer() function is called each time a comment is posted.