Have you already pressed the collapse arrow on the top right side of the widget? Sorry, I'm not sure if that's what you're looking for when you say "hide", haha. I'm super new to forum websites.
EDIT: Wait, what theme are you using? I'm using Route 167+ and the arrow is visible, but on themes like Chromatic the arrow is either hidden or the same color as the top bar. The button is still clickable, though. You see where the "report" button is on every post? The arrow is around that area on the Discord widget.