How Admin can logout other users?
If you go to admin/user/user, you can block the user, which will cause them to be logged out.  After that, if you want them to be able to login again, you can just immediately unblock them.  Also, if you as an admin change a user's password, they will be logged out and will have to use this new password to log back in.  

If you want to get fancy, you could do something like this in a custom module as well:

function YOURMODULE_user_operations($form_state = array()) {

  $operations = array(
    'logout' =&gt; array(
      'label' =&gt; t('Logout the selected users'),
      'callback' =&gt; 'YOURMODULE_user_operations_logout',
    ),
  );

  return $operations;

}

function YOURMODULE_user_operations_logout($accounts) {
  foreach ($accounts as $uid) {
    sess_destroy_uid($uid);
  }
}


which would add a "Logout the selected users" option to that menu as well.