If you have uBlock Origin you can right click in that part of the sidebar and do "Block element". This will enter the "element picker" mode and in the bottom right you can see the cosmetic filter you are creating. Here "###hot-network-questions". Hit create to hide the block you don't want to see.
If you have never used that feature, it's great! It allows you to visually pick and hide unwanted blocks, like say, the Youtube sidebar. To unhide the blocks you can go to the main icon of uB and at the bottom of the menu you have "toggle cosmetic filtering for this site". To remove that specific filter entirely, you can go to uB main dashboard > My filters.
