How do I turn off access to Plex Media Server from a Roku?

This has been troubling me and I'm not entirely sure what is even going on, but first, a bit of background.  I'm running a Plex Media Server on ubuntu on my local network.   It is not shared externally and is only accessed from local devices.  The primary way we access it is through the Plex app on our Roku. There is no DMZ and no port forwarding from the internet to the server.  I've got a Firewalla Gold, if that matters.

Here's where the problem comes in.  I'd like to be able to turn off access to Plex at certain times to keep small humans from getting up early and watching movies.

So, I figure this should be easy.  I've already got rules to disable all internet traffic to the Roku at certain times.  These work well and all other apps like netflix and disney+ are indeed disabled.  The problem comes when I try to turn off access to my plex server.  

Literally NOTHING that I do seems to be able to kill the Roku's ability to access the plex server.

Here's a list of what I've tried.

1. Blocked traffic to the server's IP address from the roku.

2. Blocked ALL traffic coming into the server's IP address

3. Blocked all of the ports used by plex going into the server.  (I know this one works because when the rule is active, I can no longer access plex from my phone or other devices).  I've tried these rules using local and remote ports.

4. Blocked ALL local network traffic in and out of the Roku. (This last one makes no sense at all that the Roku can do anything with all local network traffic blocked, but there you have it).

Nothing I do seems to have any effect on the Roku's ability to connect to and play media from plex.  I've had to set up a job to stop the plex service on my server, which is ridiculuous.

What am I doing wrong?  What should I try?  This makes no sense to me.

Thanks in advance for reading.