Here is a good article which may help you: https://docs.themeisle.com/article/32-how-to-change-the-page-template-in-wordpress

if you still cannot find an option to make the page full width, please contact your WordPress theme support and ask them to help you. If they don't help, you should change the theme to one which supports "Full width" page template.

Edit the page with the shortcode of wpForo. Go to Page Attributes. In there probably you will see Full width or other templates for THAT page.

Can I be more descriptive? I don't understand completely, I couldn't find those settings.

That setting is depends on your current active WordPress theme. So if you cannot find it you should contact to your theme developers and ask them how to set the page layout "Full Width" and ask them why they don't have Full Width page template.

In other words, this issue has not any relation to wpForo. wpForo is loaded in the page content, and it doesn't control the page layout and sidebars. You should set it in the edit page screen using the theme options.

This cannot be done from wpForo side, you should edit the forum page and check your page templates, if you find some template with "Full width" or "No Sidebar" choose it. Also, there maybe an option to disable sidebar. All depends on your theme's page templates.

Click the Edit Page on the top admin bar and try to find something to disable the sidebar.