Someone has signed into my Apple TV how do I sign them out

What is your model Apple TV box?

Identify your Apple TV model - Apple Support

Find your Apple TV model number - Apple Support

What is its version system software?

For legacy devices [A1378, A1427, A1469]: Sign out for the current account, then sign in again using the wanted Apple ID account.

For current models devices [A1625 or later], tvOS 13 or later: See if multiple users are registered. Select their account and sign out for that. If your account is also listed, then switch to that through the User Profile Switcher in Control Center. Switch users in the Profiles tab on Apple TV - Apple Support

You may need to change the password for your Apple ID, please make it complex as you should also be doing for your Apple TV apps.