It does allow you to block channels. It's the option "Don't recommend channel".
This seems to be only available on the home page, but thank you I didnt know this exists. Do you know what the option on the video page itself does "Not interested"?
You can block channels. It's in the same three-dot menu that lets you remove individual videos.
Quality of channels can go up and down though? Surprised if disliking a couple of vids from a channel isn't enough to prevent getting suggestions for that channel?