With the PRO accounts you can enable an option in the settings section of the Pro Control Panel to make your stat pages private. When this option is enabled you will be prompted to login using your PRO username and password before you can see your stats. NOTE: if you are logged into the Pro Control Panel you are automatically logged into your stats pages so you may not be prompted to log in. You can always test this by closing your browser (or clearing your cookies) and going directly to your stats pages, you will be prompted to login if you enable private stat pages.
It is not possible to make the free account stat pages private.