AppleTV Repeatedly Asking to Verify Unknown Email

Someone with physical access to your Apple TV box must have entered that account at some moment in the past, and installed an app, and later removed the account. (It can’t be done remotely.)

Now that app wants to auto-update, but it is still linked to that old account for App Store services.

To fix this, remove the app from the device. You may re-install it using your own account, if needed. If the message doesn’t mention an app name, then you may have to replace multiple apps, until you find the offending one.

To start over with a clean slate, you may consider to reset it to factory settings. Which should have been done before you got it, or else when you started using it. Reset or restore your Apple TV to its factory settings - Apple Support