Along the line where you can see the number of votes and number of comments, there is a vertical bar of 3 dots. Click on it and a menu appears. You can block the user from there.
Click the users name. (Or just go to https://lemmy.world/u/username) Near the top (to the middle right on the desktop web interface) there'll be two buttons "Send Message" and "Block User". You want the second of those.
I don’t know why it’s happening. I looked and it doesn’t seem like I’ve selected it. I wonder if I’ve selected it on a different browser And i won’t update or something