Have you already selected "Not interested" on the video? You may also want to try removing related searches or the video itself from your history, here's how: https://goo.gl/mfD7rf
If you're using a shared computer/device, consider signing out of any Google services when not in use. It is possible that you're still signed in while someone else browsed through YouTube. Also, clear your watch history and let us know if it's otherwise: https://goo.gl/y6YGpz