Have you tried actively rating or thumbs-downing stuff you've finished instead of just letting it sit there marked as watched? I've noticed that on YouTube, if I give a thumbs down to something I've already seen, it stops showing up in recommendations way faster than if I just ignore it. The algorithm seems to treat "I watched this" differently from "I watched this and didn't want to see it again" - marking something watched just tells it you've consumed it, which can actually make it recommend similar content even harder. Try being more aggressive with the downvote button on completed shows and movies, and you should see them cycle out of your feed pretty quickly.