Creating a PIN code prevents only unauthorized users from making purchases and adding channels to their Roku devices. We recommend removing the channel, the Roku Pin code will prevent unauthorized users to add the channel back and accessing the channel.
You can always block the www.youtube.com domain in your router settings which will then prevent access throughout your entire network. You need to access your router's settings and then look within there. Sometimes under Parental Controls or Security. Different routers, different areas.
Do a web search for "block youtube at router level" or the like. There are many different ways to block content from specific devices, entire network, etc.
Even if you block at your wireless network, they can still acess if they have mobile phone (assuming not connected to wireless network) so you will have to disable there also in the settings or parental control app.