Before you refer to this: I have tried installing Onavo as suggested here, but that did not allow me to do what I wanted (at least the current version of the app doesn't).
My Wi-Fi is not the most reliable, and it sometimes simply dies for a while, causing my phone to switch to data usage. Of course this always happens when I'm watching a video on youtube on my phone, and I only notice afterwards that this has happened, causing my mobile data usage to skyrocket.
What I would like to have is some way to completely prevent the YouTube app from ever connecting to the internet if I am not using Wi-Fi at the time. I have already checked the box that prevents the app from using data in the background, but that is obviously not enough.