Hi Patrick,
Thanks for your reply.
After I submit my IPF to the site, status will be "Submitted" and till the 1st level approval the status will be "Submitted".
So anyone can change/update the form in the above status and after 1st level approval form should not be change/update by anyone.
This is my requirement and i understood that, Event Receiver will do this requirement correctly.
But i am surprising that, why this below code is not working,
properties.Status = SPEventReceiverStatus.CancelWithRedirectUrl;
properties.RedirectUrl = "/_layouts/ListItemUpdate/ValidationError.aspx";