Greasy Fork is available in English.
Hover a thumbnail on youtube.com and click an icon at the right: "Not interested" and "Don't recommend channel"
< Valutazione su YT: not interested in one click
Still works for me, I use it every day, so it's either caused by another userscript (try disabling them) or youtube is experimenting as usual with random audiences.
I can no longer get this to work