As I said in the comment above, I was doing that, it was kind of working although I wouldn't say well. But then it totally stopped working - "don't recommend channel" stopping working and I think "not interested" stopped working too. The only solution was to clear my watch history and start over.
If you search you'll find this is extremely common - I think there was even a HN post about it a few weeks ago.
If your recommendations are currently absolute trash it might even be best to clear your watch history altogether and start from scratch.
But FWIW... it's absolutely worth it. The YouTube recommendation algorithm, when trained, works REALLY well and is a gold mine of useful content.