It takes a couple clicks to tell YT to stop recommending (1) an entire channel, or (2) a certain type of video (I don't like this video).
i do this all the time when cop body-cam video start taking over my YT recommendations -- all because i clicked on one video that had to do with some law enforcement topic. So, I just turn off entire channels from the recommendations.
I've actually got some channels that I subscribe to that i told YT to stop recommending, because the Al-Gore-Rhythm was recommending every video from those channels back to the beginning of the world whether or not I already watched it.
I can understand YT offering me the reason "I already watched this video" in case i watched it on another device and wasn't logged into my own account. but I get tons of video recommendations that visibly show (red line at bottom) I already watched it. So, i have to tell then to stop recommending stuff i watched because they think already watching a video doesn't mean you don't want to watch again? Are they that low on relevant content that reruns are now the norm?
If you don't bother to tell Al to stop with the Taylor Swift content, it doesn't matter if or when you clicked on any Swift videos.
I'll usually just ignore the recommendations I don't like, then when I have time on the weekend, i'll scroll through the ones I watched already or don't like and tell YT to stop recommending them.
It seems to work pretty well. My pet peeve is when I do a web search for a product to get more info, decide it's not worth buying, then sudden'y half my recommendation list is about that product.