You click the 'reply' button, copy the URL from the video (it usually shows when you click the 'share' symbol below the video on Youtube) then click the little film icon next to the smiley face in the above right and paste the URL in the box next to File/Url.
When you block a person, they can no longer invite you to a private message or post to your profile wall. Replies and comments they make will be collapsed/hidden by default. Finally, you'll never receive email notifications about content they create or likes they designate for your content.
Note: if you proceed, you will no longer be following .