If you have a router you control, many routers allow you to take away internet access from a device while keeping it on your local network. Some (all?) Asus routers can do this from their UI.

This won’t help with devices that require 24x7 internet access, but it’s great for things you want to access the local network but don’t trust not to send info to a third party. (TVs, music amps with built in streaming, home surveillance systems [1], etc.)

Also handy for briefly turning on Internet access for software updates or one time activation.

[1] while making a surveillance system available online safely and with software you control isn’t hard, it’s not trivial. Turning Internet access off for your cameras without a plan will mean you can’t monitor your home or get alerts when away from your local network.