You can’t unpublish your website, but there are several workarounds.
Disconnect the domain and reconnect it once you're ready to publish your website.
Delete the published template and continue working on its backup copy.
Create a landing page and hide the rest of the pages you're still working on.
Disconnect the domain
To disconnect the domain, go to the site Settings > Domain, and click Disconnect domain:
Delete the published website template
If you delete a published website template, it will go offline. However, all of your content will be lost this way, unless you generate a copy of the website template before deleting it:
1. In your Dashboard, hover over the thumbnail of the website that you wish to unpublish (delete) and click on the settings wheel.
2. Click Duplicate site. You will then have two versions of your website, a published one and a not published one.
3. Delete the original (published) website that you wanted to unpublish and keep working on the backup copy.