If you tag a series and then indicate that a user should be restricted from that tag, then the parental control cascades down to episodes underneath.
Tested this and it did not work. Blocked content with tag "private," marked a series with that tag, did nothing to block. Episodes were still showing up in Next Up queue.
I'd rather not change the rating. Then users that are allowed to access it would see incorrect information. Otherwise I would just change it to XXX, but it's not supposed to be. Just want to disable this one series with no visible change in metadata.
The only way I was able to accomplish this was to add the tag to all episodes. Thankfully that was only like 10 right now, but it was still painful.