Or is there a process to turn a "smart tv" into a dumb tv? (per brand/make/model)
Devices can refuse to function without a network connection, and the profits to be made from simply including a cellular connection of some sort for ad refresh + tracking are too high to be ignored forever.
So this isn't a reliable, long-term solution.
(ok, most TV vendors won't bother to fix and most video sources ate somewhat trusatable not to exploit ... especially as an exploit can't do much unless it is connected to a network)
Some of them allow updates via USB stick.