Revoke permissions from a user role.
$rid: The ID of a user role to alter.
$permissions: A list of permission names to revoke.
user_role_change_permissions()
function user_role_revoke_permissions($rid, array $permissions = array()) {
  // Revoke permissions for the role.
  db_delete('role_permission')
    ->condition('rid', $rid)
    ->condition('permission', $permissions, 'IN')
    ->execute();
  // Clear the user access cache.
  drupal_static_reset('user_access');
  drupal_static_reset('user_role_permissions');
}