Open YouTube and sign in to your account
Go to the channel of the person you want to block
Click the About tab on their channel
Click the flag icon
Select Block user
Confirm the block
To block comments from a user, go to one of their comments
Click the three dots next to the comment
Select Hide user from channel
Confirm the action
To unblock a user, go to their channel or a comment
Click the flag icon or three dots
Select Unblock user or Show user
Confirm the change