Just use a website blocker.
Most good website blockers allow you not only to block entire websites but also specific pages or URLs, including YouTube channels.
Simply add the URLs of the channels you want to block to the block list, and they’ll become inaccessible.
If you're doing this for your child — for example, some YouTube channels are inappropriate for kids. It's best to choose a blocker with password protection. One good option is Cisdem AppCrypt, which lets you block specific YouTube channels and also prevents the settings from being changed without entering the correct password.