Not certain what you are describing. Roku devices cannot stream or cast their content to anything that isn't connected to the HDMI cable on the Roku. It is possible for someone on the same network to use the Roku app on a phone or tablet and control your Roku, but other than The Roku Channel they can't see any of the content on your Roku. You can prevent a networked device from connecting by going into the Settings menu and find the setting for allowing remote devices to connect and disabling it.
But they can't see on their TV what you are watching on yours unless there's an HDMI splitter on the cable and it's feeding both TVs.
Dan Roku Stick (3600), Ultra (4640), Ultra (4670), Ultra (4800), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.