How to remove the HTML tags and attributes option from comment forms in WordPress
Sometimes you may prefer to avoid your visitors using html in their comments. WordPress usually permits some html tags and attributes, listing them above or below the comment form.
To remove the array of these tags and attributes you can just add this line in you style sheet (style.css) at the folder of your theme.
If you prefer to edit the php code of your theme, then search for
<?php comment_form(); ?>
and replace it with
<?php comment_form(array('comment_notes_after' => '')); ?>