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
I use it daily and it works for me. I guess youtube as usual is enabling different experimental changes for random users.
Try installing older versions of the script or go to youtube settings -> Playback and performance -> and disable Video previews.
Stopped working for me recently, not sure when it broke.
Any chance of an update?