The only way I've managed to turn it off is by changing the language of youtube to be English (I've had it in my native language before).
(Bad for two reasons: (1) badly translated titles, sometimes hilariously so, and (2) the algorithm recommends me videos I have already watched. I guess (2) is because I don't usually watch the last 20 seconds, the static "my last video was", whatever it's called, so YouTube must think I haven't watched the video. Most times, the red gauge below the video thumbnail shows half-full even though I've watched the video minus these 20 seconds. There's still room for improvements...)