This page, 'RecentActivity', gets special handling on PBwiki — its contents will be shown as part of the sidebar drawn on all standard pages. By deleting this page you can remove the sidebar display. If all 3 special pages (SideBar, QuickStart, RecentActivity) are deleted or otherwise don't exist, the sidebar is not shown. In addition, the sidebar is not shown when viewing these pages themselves, like you are doing now.