Triggered before a user object is saved. You can use this event to prevent the changes
from saving by throwing an exception in the event handler.
public function subscribeEvents()
{
Backend::$events->addEvent('core:onUserBeforeSave', $this, 'before_save_user');
}
public function before_save_user ($user)
{
if($user->id == 1)
throw new Phpr_ApplicationException("This user cannot be edited.");
}