Site Configuration
How do I not allow guests to purchase orders on my site?
By default, guests (customers who do not register on the website) can purchase orders from your website.
In order to turn this off, click "My Store" under "Site Configuration".
About halfway down there is a field titled "Allow Guests Accounts". Click once to highlight and bring up the right side information page, then press edit and switch the value to "false". Press update and you are complete.