If you are using a Desktop then follow the given steps below: -
Open the channel that you want to block.
Go into the "About Section" or click on More to visit.
Click on the "Report User"
The four options will appear on the screen, either you can hide/block the channel with the first option. Or directly report for any of the recent causes available below.
If you are using a Mobile then follow the given steps below: -
Open the YouTube application on your device.
Select the channel or a single video of the particular channel you want to block.
Click on the three dots at the lower right corner of the video.
Now tap on the "Don't Recommend Channel".
Hence this is the simplest way to hide or block unwanted YouTube channels. Hope this helps!