I don't know him personally, and don't listen to his music. His life and existence does not effect me in any way, whatsoever.
However, there's this whole attitude on Youtube videos that people feel the need, no matter what they're watching, to make a reference to him be it in the likes/dislike comments, or by comparing the content of the video to him, saying that in some way it is better than him, even if it is completely unrelated. This is spam. This is why Youtube has a button you can click on comments called "mark for spam".
If you are not watching a video about or of Justin Bieber on Youtube, you have no real reason to bring him up, or comment about him unless the video is in some way related by at the very least the same musical genre. Even if you absolutely despise him, by making these comments you are simply drawing more attention to him than you need to, and the comments themselves are spam.
Either way, next time you're watching a Youtube video and you notice these comments about Bieber and the video has nothing to do with him, don't get into a fuss and comment back. Simply click the thumbs down picture beside the comment, click to mark it for spam, and continue watching the video you were there for in the first place. Don't forget to spread the word too. The more people doing this, eventually the comments will all stop and slowly disappear.