"Please don't watch that movie yet."
Because the way that は sets the topic of discussion can have a useful contrastive effect. It's like you started with a whole shelf of movies, zoomed in on that one in particular and said please not to watch it yet. There's a bit of an implicit suggestion that maybe another one would be okay (without actually saying that).
With を maybe it's not just that particular movie, perhaps there's something else you want the listener to do first.
I may be wrong, but I understood the は particle as a topic marker, indicating what the sentence is about, が as subject marker, showing what takes action, and を as object marker. So sometimes the topic could be the subject and the object, and could also be neither, like hehe: わたしは花が好きです So to answer your question, I think it could be both は and を. But I am no mother speaker so I'm not sure.