Below are the Item Deleting event receiver code which prevents user from deleting an item with the notification.

/// <summary>
/// An item is being deleted.
/// </summary>
public override void ItemDeleting(SPItemEventProperties properties)
{
base.ItemDeleting(properties);
properties.Status = SPEventReceiverStatus.CancelWithError;
properties.ErrorMessage = "This item cannot be deleted, Please Check your admin Team";
properties.Cancel = true;
}
If you remove the line "properties.Error Message = "This item cannot be deleted, Please Check your admin Team"; the code still works but there wont be any notification provided to the user.!!