A website that catalogued the misbehaviours of the various smart TV operating systems (and the easiest methods of defeat) would be handy here.

E.g. Some TVs will honor wifi off setting. Or alternatively setting the TV to use the Ethernet port.

Or if it needs something on the other end, set up old underclocked Raspberry Pi as a basic router/DHCP server that connects to nothing; power it with TV's USB port.

If you've got a fancy router, connect it to your network with a fixed IP and firewall deny all packets from/to its IP.

If you've got a fancy AP, set up an alternative SSID that connects to an unused VLAN or otherwise routes to nowhere.