Are you using the Product Reviews app by Shopify?
With this app, your customers are not invited to log in to leave a review, however you do have the option to manually publish these reviews instead- some more info on this here. There is also the option to connect the Product Reviews Add-on which emails each customer directly after they have made a purchase, inviting them to leave a review. You could make customer accounts mandatory if you like, you can do this by going to Settings > Checkout and ticking the 'accounts are required' box here:
If you wanted to lock certain sections of your store, there are a couple of apps here that can help you out:
The full range of apps is here if you'd like to check them out! A lot of these apps offer similar services, so I'd recommend shopping around and reaching out to the app developers to make sure they can fit your business's needs. You can see the app developers contact details if you scroll to the 'Support' section of the app listing:
Were you noticing that people who have not purchased from you were leaving bad reviews? Or did you want more control over who can leave the reviews?
Social Care | help.shopify.com