If you mean to the menu bar with "forums" "members" "register" and "login", You can disable it easily.
Just go to you control panel, appearance->menus->choose the wpforo navigation menu->uncheck the "wpforo menu" on the bottom of the page and hit save.
Hopefully I solved your problem ! 🙂
Just saw the screenshot and the thing you want to remove is about the theme, Check the customize under appearance, Search for header or something like that.