how to cancel a delete in django signal

Is there a way to cancel a deletion of record using django pre_delete signal?

example:

def on_delete(sender,**kwargs):
  if not <some condition>:
    #cancel the deletion
 # else continue with the deletion
pre_delete.connect(on_delete,sender=MyModel)

and another question is there a way to say to a model "that before changing a file delete first the original file" because right now this is what I do(see code below) and I'm not sure if this is the best way to do it.

def on_save(sender,**kwargs):
  obj = kwargs['instance']
  try:
    id = obj.pk
    # find the file
    original_file = sender.objects.get(pk=id)
    # delete the original file before uploading a new file
    original_file.file.delete()
  except ....

pre_save.connect(on_save,sender=ModelWithFileUpload)

(in django 1.2 they automatically delete the file on change or on delete but in django 1.3 they removed this feature)

Thanks in advance