Disabling wifi password sync between Apple TV and iPhone
You need to disable Keychain sync.
This will affect all keychain items; you can't selectively choose which items will sync.
On the Mac, it's System Prefs &gt; iCloud &gt; Keychain
iOS is Settings &gt; [Your name] &gt; iCloud &gt; Keychain
In all honesty, I'd avoid this. Change a password on any web site &amp; you'll have to manually change it on every device that needs to access the same resource; or prevent passwords from being saved entirely.
The other thing to perhaps consider is that WPA already uses a different (pairwise) key per device, even if it shares the same password (PSK). Adding one password per device is no more secure, at consumer-level, merely more difficult to admin.