In the app, go to "More" or "Settings" and select "Sign Out".
On the website, when viewing a video, at the bottom right you will see "Signed in with [Your Provider]. Mouse-over this and click "Sign Out".
On iOS and tvOS devices, if the app won't sign out, go to the device's Settings > TV Provider > Scroll to the bottom and choose "Sign Out".
Unfortunately, we do not have a way to sign out of a device remotely. You can try changing your TV provider account login, or see if your TV provider is able to remotely sign out any active sessions.