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Pretty simple:

Settings>
Users and accounts>
Choose your user name as current user and you should see apple tv accounts.

click icloud


now you should see icloud photos, shared albums and my photo stream. Set all to off.

Now if anyone tries to turn it on it asks for your password.

There you go I hope I answered your problem.
 
This no longer works and iCloud photos can be turned on yet again without a password confirmation. If you try turning on iCloud photos from the actual app it will ask you for a password. If you go into the settings menu and then to your iCloud account you can just click on without a password prompt. Going back to the apple photos app on the tv confirms the photos are available.

How the hell can Apple keep screwing this up? It's like the 3 or 4th time in 5 years they have fixed it and then the problem returns.

*EDIT...I just went to my other Apple TV and it does prompt for a password there in both locations. Both Apple TV's are running the latest OS. Pretty comical..
 
To me this is a security flaw. (Especially considering it can be activated without a password) I’m not sure everyone out there realizes how vulnerable their pictures are.

Just think about all the kids unsuspectingly sneaking around, jealous spouses or just a curious friends or roommates could have access to a whole lot more than one is willing to share!

Can someone who’s on a non-beta release confirm this is also the case on tvOS 11?

Kids I get... that is why any sensitive photos are not part of my iCloud or are hidden in Photos. Jealous spouses??? IF your spouse is that insecure, you have more problems than photos. Curious friends??? Seriously? people you know just invade your privacy when invited over?? If you live with a roommate, I would suggest a specific account just for the main TV.
 
Was googling around for similar concern... Here is what I found. Don't blame me if you don't like what Apple provides here... I am merely here to share that for those folks that have not seen/discussed this page:

https://support.apple.com/en-us/HT204570

How do I turn off iCloud Photos on only one device?
You can turn off iCloud Photos on a single device when you follow these steps:
  • On your iPhone, iPad, or iPod touch, go to Settings > [your name] > iCloud > Photos, then turn off iCloud Photos.*
  • On your Mac, go to System Preferences > iCloud. Next to Photos, click Options, then deselect iCloud Photos.*
  • On your Apple TV 4K or Apple TV HD, go to Settings > Accounts > iCloud. Under Photos, press Select to turn off iCloud Photos.*
If you have Optimize Storage turned on, you might not have all of your original photos and videos on your device. To download the original photos and videos on your iOS device, go to Settings > [your name] > iCloud > Photos and select Download and Keep Originals. On your Mac, open Photos, choose Photos > Preferences, then select Download Originals to this Mac. Then you can turn off iCloud Photos. You can also select the photos and videos that you want to download from iCloud.com.
Before you turn off iCloud Photos, you might want to make a copy of your photos and videos.
 
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