Customising YouTube recommendations
There's a "don't recommend channel" option [on YouTube] if you click the 3 dots button on a video. I use this liberally for all sorts of reasons and it makes my recommendations much better.
– https://news.ycombinator.com/item?id=31324917
A much better approach is to go to your history and delete it from your history (if you're signed in) this will change your recommendations so much harder.