"There is a setting under settings>system>advanced to disallow control by network apps, but I think that is just to keep them from starting if they aren't already running. If you are already running Youtube or Netflix they may still accept casting commands since that is just the way the app works. You could remove the offending apps, or just don't use networks you can't control."
That system setting doesn't do anything, casting can still hijack your Roku. Deleting the apps that are being cast to or not using the same network that the casting devices are using are not acceptable solutions.