I wonder, is it even possible to truly delete a movie? I am surprised Batgirl didn't leak. How do you even prevent that? It's just bits.
Keep the working copies on computers in a special lab with no external network access.
Restrict access and physically search the people leaving the lab.
Not fool proof, but it will reduce leaks by a lot.
But yeah, once you send out thousands of review copies, there's no stopping the leaks.