Do not recommend channel doesnt work. It only pauses recommendations for a few weeks. I've selected do not recommend on channels several times and they still sneak back in.
I've heard this from a couple other people, though, which makes me wonder if the alleged "don't recommend channel" button is hooked up to different API calls in different UIs...
Just checked, and I don't see it in the suggestions under a particular video, only on the main feed. I don't know if they're hiding it or what.