Remove choice for user delete from Roles which can adminster the Users
I would start by creating a custom module and try using hook_form_alter.

function MYMODULE_form_alter(&amp;$form, $form_state, $form_id)  {
  switch ($form_id)  {
    case 'user-multiple-cancel-confirm':
        // ALTER STUFF.. user dsm($form) to show available fields.
        break;
   }
} 


Using 

global $user; 
$roles = $user-&gt;roles; // Array with user assigned roles


you can check the array for the Supervisor role.