Youtube previews not blocked

My guess is that there is not a solution for this issue, but I'm keeping fingers crossed that there may be.  I have youtube blocked on my son's computer with Norton Family, which works fine when he tries to go to the website itself.  BUT ... I have discovered (after he did unfortunately) that links to youtube videos will play in "preview" mode within gmail messages, and within google docs.  Within a google doc he's even able to open a mini-viewer and watch the video.  If he tries to open the video in a new tab in a browser, it is blocked.  But he's already watched it at that point, so that isn't helpful.

I can't find a setting in google accounts to disable previews, and it seems that Norton Family doesn't recognize previews as actually visiting a blocked site.  Is there a Norton fix to this issue?