Log out of Apple TV
The easiest way is probably to change your Apple ID password. However, you would then have to sign in to all your own devices again using the new password.
Go to https://appleid.apple.com/ log in with your AppleID and password. In the next page, scroll down and you will see every device you have signed into with that AppleID. You can remove any device from the list right from within that interface by clicking on the device you wish to remove and confirming the removal.