If you don't mind voiding the warranty, open up the TV and find the WLAN and/or cellular modem, and snip off the antenna/break the PCB trace used as an antenna.

If you don't mind voiding the warranty, open up the TV and find the WLAN and/or cellular modem, and snip off the antenna/break the PCB trace used as an antenna. In some cases it's probably attached with a tiny coax connector that you can disconnect. No antenna, no signal. Just make sure to get a TV that doesn't need online activation to function.