Login to your admin control panel.
Go to System > Settings then click on Edit.
Go to Options.
Under the section Reviews, you will see three options: Allow Reviews, Allow Guest Reviews and New Review Alert Mail.
If you disable Allow Reviews, even if Allow Guest Reviews is enabled, no one would be able to post reviews on your website. If, however, you enable the Allow Reviews, with Allow Guest Reviews you can toggle whether guests will have the right to review your products.