In the 'dokuwiki' template, near the top of main.php, you will find:
$hasSidebar = page_findnearest($conf['sidebar']);
$showSidebar = $hasSidebar && ($ACT=='show');
Underneath that include the following code:
global $INFO;
if($INFO['id'] == 'start'){
$showSidebar=false;
}
So now you will have:
$hasSidebar = page_findnearest($conf['sidebar']);
$showSidebar = $hasSidebar && ($ACT=='show');
global $INFO;
if($INFO['id'] == 'start'){
$showSidebar=false;
}