Before and after posting a video, you can make that adjustment. Before you post it, there is a section asking your preference about comments. If you turn that off, people won’t be able to leave comments on your video. After posting it, you can make the adjustment on YouTube Studio. There is a Menu on the top left of the page. There, click on Content. Then click on the video’s thumbnail. Under “comments and ratings,” you can change the comment settings. After you click Save, if there are any old comments, they will be hidden. And no new comments will be allowed.