You could use the 'protected folders' feature of your hosting account (if
such a feature is available) through your control panel on your hosting
account.
Windows calls it "Protected Folders" and Linux/Unix use the 'htaccess' and
'.htpasswd' feature. The functions are both the same.
Apply it to your 'root' folder of your site, and no one can get into it
without the password. Contact your host about this to see if they offer
such features as part of their hosting packages.