For the description, try setting it to a space. Yoast currently has a check in place for various empty values, but a space will get through. This I found yesterday when testing some things.
WordPress seems to handle single space meta fields as needing to be emptied and just saves it without that space, or at the very least, doesn’t output the space in the backend.
I haven’t tested, so I can’t guarantee that Yoast won’t just try to give an empty meta description for those pages though. If it works the way I think it will though, it’ll determine the description you entered to be empty and generate one for you again.