WordPress.org

I am unable to delete the temporary opted in users. I get this message: Cannot load wpsb-opt-in.php

The wpsb-opt-in.php file must be in the /wp-content/plugins/ folder and not in a subdirectory.

Is it possible to show the form in the body of a page rather than on the sidebar?

Yes: to have the newsletter widget inside a post you must enable PHP execution inside posts. First install this PHPexec plugin:

http://wordpress.org/extend/plugins/exec-php/

then write this code in your post:

<?php wpsb_opt_in(); ?>