You page is not full width page, the right section is a sidebar section. It seems the theme doesn't have such template in Edit Page > Page Template adminbox. if it does have such option please choose it.

This problem also comes from widget cache. Please navigate to Dashboard > Appearance > Widgets admin page. Find the default right sidebar. Put some widget in this sidebar and check the forum page. if you see that, just delete this sidebar. This should reset the cache and if the theme is smart, it'll remove the sidebar section.