Basically Samsung TVs at least used to grab any open Wifi to connect to if you don't provide them with credentials to your own. Also there is no way to fully disable the wifi.

The solution was to set the connection to manual configuration and set all addresses to 0.0.0.0.

Of course this is not an issue if the only network your TV can see is your own, more of a problem in apartments where there might be 20 wireless networks available at any time.