If you don’t want users to post HTML in comments on your site, you can post the code below into your WordPress theme’s functions.php file:
/* Disable Hyperlinks in WordPress Comments */
remove_filter('comment_text', 'make_clickable', 9);
add_filter('pre_comment_content', 'strip_comment_links');
function strip_comment_links($content) {
global $allowedtags;
$tags = $allowedtags;
unset($tags['a']);
$content = addslashes(wp_kses(stripslashes($content), $tags));
return $content;
}