How to disable it?

set it's status to 0 in the database

you can also remove the folder then use paths to get to the admin area. This will mean you site will be stripped of a theme until you get back to the admin area and turn on a new theme.

Sorry for the long delay in replying to this.

You should have read the README.txt.

The best option is to do the following:

1) Copy page.tpl.php from the "launchpad" directory as page-offline.tpl.php
into your own theme's folder
2) Copy settings.php from the "launchpad" directory into your theme's folder
3) Add the functions from template.php (in the "launchpad" directory) to your
own theme's template.php
4) Copy the directories css, images and js (and their contents) into your own theme directory
5) Install ThemeSettingsAPI module
6) Add the FeedBurner account information in your theme admin config page
7) Set your site as offline to see it working

Hope this allows you to use it.

Another solution is BEFORE activating this theme AS IS, set a different theme for the admin part to allow you to access the admin section of your site.