Modify the following to your theme's config.php.
// The pagelayout used for reports
'report' => array(
'file' => 'report.php',
'regions' => array(),
'options' => array('nofooter'=>false, 'nonavbar'=>false, 'noblocks'=>true),
),
Setting noblocks to true stops the ability for anyone to add a block to the grader report page.