If you are an admin on an organisational Caption.Ed license, here's how you can block users from using the browser extension to caption content hosted on certain domains
Select "Settings" on the left-hand menu, then select "Organisation Admin".
Scroll down the page till you see the "Captioning Source control" menu.
Select "Add new" at the bottom of the menu.
Enter the URL of the website you wish to block (e.g. www.netflix.com).
Click "Save" to confirm the change.