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
Block YouTube
Edit your block and add youtube.com under the websites tab of the block.
4
Add the exception
In the same block as step 3 (and all other blocks that include youtube.com), add the correctly formatted URL to the website exceptions tab of the block(s).
NOTE
You can also block specific YouTube channels using a similar method described here.