It is always recommended to use latest version. If possible, update to latest version.
In your version, you can use add_meta_box to show checkbox in announcement add page.
to prevent email sending you can use
update_post_meta( $message_id, '_fep_email_sent', time() ); in your save function.
Again, if possible, update to latest version so that you can easily achieve this. I can provide you full code if require for latest version.