If you want to exclude certain products from being reviewed or exclude sending review requests to specific customers, go to the Exclusions tab under Store Settings.
If you want to exclude review requests across the board on your website - including not showing the “write review” link in the widgets - go to the Exclusions tab under Store Settings.
To get to Store Settings
Go to Configuration → Settings
Then click on Exclusions
There are multiple product or customer attributes that can be used for exclusions.
Here is a video on how to do a product exclusion in your store: