How to

Block a YouTube channel

1

Find the channel URL

You can do this by watching a video from the channel and then clicking the uploader's name at the bottom of the video.
2

Format the URL

Usually, the channel URL will look like: youtube.com/@username. Remove the @ sign and you now have the correct format: youtube.com/username.
3

Add it to a block

Edit your block and add the correctly formatted URL to the websites tab of the block.
NOTE
You can also block all of YouTube but only allow certain channels using a similar method described here.