To mute forums, go to your Preferences (click on your avatar at top right, your username at top left of the popup, and then Preferences.) Select Forums at left (or in the right-side dropdown box on mobile). Add the forums you don’t like in the Muted box at bottom. As the legend reads, “You will not be notified of anything about new topics in these categories, and they will not appear on the categories or latest pages.”
Or, toggle your preference using the bell icon at top right of any forum’s page (top center in mobile).