It depends on the comments form. If the hook call by means of which captcha works ('after_comment_field' or something like this) is present in the file comments.php, you can change captcha positioning by moving this hook call. Please find the file 'comments.php' in the theme and change position of the line
do_action( 'comment_form_after_fields' );
or any similar line - place it under the Submit button.
In case there is no such hook in the comments file of your theme, then, unfortunately, this option is not available.