What do you mean hijacking it? If your on the same network - then yes they would see it. If you can not create a private wireless network, ie different security and SSID then anyone on that network would be able to see/use the chromecast
What you could do is get your own wireless router and put your self behind the network your sharing..
So you have this
internet -- wirelessrouter -- switch
its sounds like. So you could do this
internet -- wirelessrouter -- switch -- yourwireless router, or you could be connected to the wireless routers lan port as well just like a switch. But vs using your new wireless router as AP, you would use it as actual nat router and lets say your current network is 192.168.1.0/24 -- set your new routers lan/wireless network to be 192.168.2.0/24
Now your devices connected to your wireless or its lan ports would be isolated from the shared network just like your current network is isolated from the internet.. And unless they plug into your routers lan ports or switch connected to your router or know your wireless password they would not be on your network and would not see or be able to use your chromecast.