Depending on what is available in the theme you are using, you can change this in the dashboard Appearance> Customize > Appearance tab to full width. However, this setting also applies to posts. Pages can be set full width if you desire a website look. Some themes will give you more variable options.
I had to decide with my site that I really wanted the option to have sidebar material on some parts of the website so I was stuck using sidebars on the forum. I used some of the neat bbPress plugins like “recent topics” “recent replies” and “search forum” in my sidebar to fill the space.