Yes. To do so log into the Control Center. On the main Control Center page, you'll see a large green ON/OFF button. Click on the OFF side to turn your site off.
When your site is turned off, visitors will see a page notifying them that your site is currently unavailable. You can change the message on that page by clicking on the Change Message link you'll see under the On/Off button when you've turned the site off. Enter a new message in the box then click on the Save button.
To turn your website back on, log into the Control Center and click on the ON side of the large red ON/OFF button.