You can make your site Private which only allows people you choose to view your site.
IF you would like to do so, go into Settings> Reading. You will see a section that says Site Visibility, click on "I would like to my site to be private..."
Here is more information on Site Visibility: http://en.support.wordpress.com/settings/privacy-settings/
If that is not what you are looking for, you can delete your content without deleting your domain. You will need to go into each section (Posts, Pages, Categories, Tags, Media) and click the box next to each post, in your case, you can click the Check All box, and then select Move to Trash in the Bulk Actions drop down menu.
Here's more information on deleting content: http://en.support.wordpress.com/empty-site/