You can delete your Joomla.com website through the Delete link, located in the options menu (gear icon) for the specific site.

You just need to confirm your wish and your website will be deleted:

IMPORTANT: Once you confirm the delete action, your website will be completely deleted and there will be no way to recover any part of it. The name, that has been used by your website, will become immediately available for anyone to start a new website with it.