Special thanks to @Gothix for explaining how to do this, as it’s a little bit roundabout. Click on your icon on the right of the top bar and click on your own name at the top of the drop-down list. On your profile page, click on the … button on the right and select Blocks on the dropdown list. On the right side of this page, type the username you wish to block into the field, and click on the username that you wish to block. Click Toggle Block next to the displayed user information.