A guest logged into Wi-Fi on a Vizio of mine and there was conveniently no way to disconnect/forget it without a factory reset back to motion smoothing hell.

Change your network name. When the TV prompts you to connect, join the renamed network. Then, rename it back so everything else can connect again and the TV can't. I can think of a few potential problems with this, but, it might work?

Or blacklist the TV's MAC address in your router settings. Didn't think of that first for some reason.

You gave me flashbacks to my Samsung washing machine that needed a factory reset after changing my SSID. Which also reset the service life of filters and liquids and such which was somewhat of a hassle. Such a dumb design not being able to change the wireless network.