You might need to strip out HTML in comments and here's how.

/** ******** ******** ******** ******** ******** ******** ******** ******** 
* TITLE:       Disable HTML in WordPress comments
* DESCRIPTION: Disallow HTML in WordPress comments, add this snippet to your functions.php file.
* 
* http://www.labnol.org/internet/wordpress-optimization-guide/3931/
*/
add_filter( 'pre_comment_content', 'wp_specialchars' );