ZyXEL → Re: USG50 Firewall Setting Question

The problem is most likely with already established sessions when the rule is activated. This is known issue that some existing sessions don't get killed right away (conclusive testing and proof required).

It should work at the time you apply.
Anyway, sometimes it fails ...

I see that with the schedule I have set up to cut off my kids internet access at night. Existing games, etc. continue to work just fine as long as the connection to the remote server was established before the scheduled firewall rule kicks in.

Yep, I've just tried it myself and the FW rules don't seem to be applied to existing sessions. Only new ones.
...I've read through CLI hoping there would be a switch to allow killing all active sessions from / to specified zones but there seems to be none.

Just realized we've had this discussion already here »USG100 - Weird (and frightening) firewall behavior

The problem is non-trivial, the perfect solution would be to have "flush existing session table" command (or similar). ... but I can't find any.
Reboot or disabling/enabling the WAN interface seems to be the ugly alternative (the interface disable/enable could be scripted and scheduled (I've not tested this))