Block a user: You can block someone so they can’t comment on your channel or interact with you in certain ways.
Hide a channel: If you don’t want to see a particular channel’s recommendations, use “Don’t recommend channel” when it appears in your feed.
For kids: If this is about keeping certain channels away from a child, YouTube parental controls give you more control over which channels and videos are available.
YouTube doesn’t generally have a simple “block this channel everywhere” button for regular viewers.