Help & troubleshooting for channels on your Roku device, including adding/removing channels, logging in to, authenticating, or activating a channel, channel-specific playback issues, assistance contacting channel publishers to report issues, and adjusting channel-specific settings.
The Netflix channel (app) offers a feature that allows you to 'cast' a video onto a supported device, which includes Roku devices. If someone uses this feature in the app on a mobile phone or tablet, this will cause the behavior you described. You'll want to contact Netflix support to see if they can provide any help in disabling this feature in their channel.
One alternate method to help avoid this capability would be to move either your mobile devices/tablets or your TV to an alternate wireless network, so that your mobile devices and TV are not connected to the same wireless network.