Find where the comment_form() function is called. Usually it will be in the comments.php file of your WordPress theme.
It might be displayed like:
<?php comment_form(); ?>
Pass an argument to the function as below:
<?php comment_form(array('comment_notes_after' => '')); ?>
The array can have other arguments as well to mod the comment form to your liking. Refer to WordPress comment form codex for all possibilities.