How can I create a program to block specific YouTube channels?

You're diving into some tricky territory! Typically, what you're describing sounds a bit like a man-in-the-middle technique, but be warned: most modern browsers and apps, including YouTube, will spot and block such attempts to tamper with connections.

Unfortunately, your filtering idea hits a brick wall due to network encryption. You can certainly block access to youtube.com from a specific device using your router's settings. However, blocking videos from a particular creator at the network level is complex because each video's URL is unique. You might end up needing a list of all their videos, which isn't practical. A browser extension that hides content from certain channels could work too, showing blank placeholders instead. However, keep in mind that users can easily disable extensions, so if you're trying to stop someone from engaging in bad habits, this might not be very effective. Plus, they'd always have the option to use their phone instead.

As the previous user mentioned, browsers reject tampered requests. A more feasible solution might involve using a Greasemonkey script to modify the request after it reaches your browser and before you interact with it.