[gibolt]

They have two options under the triple dot menu (at least on Android feed) that you are not interested in this video or this channel.

May help curate once you start getting those recommendations.

[TobTobXX]

I once watched a few videos about US politics. After that YouTube just kept spanning half my recommendations with vids about USpol. While they were addicting, they didn't really interest me. I then took the effort to flag tem as 'not interesting' (about 3x, refreshing the browser) and it works, I have gotten very few recommendations about that since then.

[ianbooker]

Then you cut out videos you were interested in the first place.

[Alphaeus]

When it asks you why you're not interested, one of the options is "I've already watched this video".

[PopeDotNinja]

I used to do that, but it was a lot of work & I didn't notice an improvement to the search results.

[etripe]

I experienced exactly the same. I suspect that since they're optimising for engagement, already having watched a video is some indication that you'll engage with it again.