Hover a thumbnail on youtube.com and click an icon at the right: "Not interested" and "Don't recommend channel"
< Feedback on YT: not interested in one click
The new 1.1.6 should fix it. I guess you were using the inline preview player, which I always disable in account prefs.
1.1.6 didn't work on update, but seeing another commenter here saying to reinstall it (delete, then instal) worked !
Stopped working, hope there's a fix soon :)