This is caused by an app running on a phone or tablet on you network called "TV Cast (Roku)" by 2kit Consulting. It uses the external control protocol to launch a channel on your device called "TV Cast (Official)" to push pictures and video to the Roku device, so when this is showing up over Netflix, it's actually forced you away from the Netflix channel to run its own content.
If you don't have the TV Cast app installed, it instead uses a system feature of Roku called "Play On Roku" that's used by the Roku mobile app to show pictures and video from your phone. That's a bit sketchy. Unfortunately, there's no way to disable use of this without also disabling the ability for the mobile app to connect.
I suspect the same mechanism may be used by other casting apps, so you'll have to check any devices on your network to remove the bad applications.
Which is exactly why I suggested that Roku change this so that it uses a similar model that Apple uses. A device is connecting to a Roku needs to be authenticated first. There are several ways this can be done that's easy for the user while keeping the Roku secure. Once a device is authenticated, the user could give the app attempting to connect permanent access so that subsequent connections are automatic. Automatic connections without any sort of authentication is just asking for trouble. If Roku doesn't fix this then it's likely more and more people will ditch it over time in favor of other platforms.