How to Remove the Website Field of WordPress From the Comment Form?

We found so many spammers try to leave their url in the field of Website, and use Name as their keywords. How to prevent the spammers doing so then? For example, how to prevent them to leave thir url? Let me share my tips with you.

Go to your blog root, open wp-comments-post.php and find
$comment_author_url = ( isset($_POST['url']) ) ? trim($_POST['url']) : null;
You can add your own url here, for example, add, the code will be:
$comment_author_url = ""; //( isset($_POST['url']) ) ? trim($_POST['url']) : null;

Done. I have tested the code successfully under wordpress Version 3.2.1