I am sorry, I did some tests and found that an empty owner field means that anybody can modify or delete the SD. I guess @Seneca has to correct a bunch of replies that stated that an empty owner field makes a SD permanently immutable. What is strange is that Maidsafe never corrected this common belief (in my knowledge).
So I think that Maidsafe should revert to previous delete implementation with a post having an empty payload. That way a SD having version 0 is a proof that the SD was never modified.
This is not perfect yet because we have to trust the app that it only generates such SD objects but this is easily verifiable and it is in the interest of the app to do so because if it is caught generating a SD with a version > 0 then all trust will be lost.
Another way would be to change implementation of empty owner field, but I suppose this is a feature needed in some cases (for example for shared modifiable directory).