Sony Bravia TV - need to prevent Chromecasting

I have a Sony Bravia (android) TV at work that is connected to a secure network. I have discovered that when the secure network is not available, the TV connects to our non-secure network and this enabled someone on the property to cast to the TV. I tried to forget the non-secure network, but it just keeps coming back, I guess because it's non-password protected. Is there any way to permanently delete a non-secure network from my available networks, or any other way to prevent people from casting to the TV? (Please forgive any misstatements or omitted info, I am about as non-technical as they come.)

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