Can't block a device. Device reappears on the network with different MAC and same "base" hostname.

I have a Cox Panoramic Wifi router, and I'm trying to block my son's gaming PC to limit his time gaming.

When I try to block the PC from the network, it keeps reappearing with a different MAC address, but a slightly modified hostname. I don't know if there's some kind of utility he's running or if it's the gaming app that keeps recycling MAC addresses and re-connecting to the network. Here's an example

• Let's say the device hostname that I'm trying to block is NEW-DESKTOP with MAC address X:Y:Z.
• I block the device by selecting and pausing it, and it gets blocked and everything's fine
• Then a few minutes later, the same device reappears with a different MAC address (A:B:C) and the new device name is NEW-DESKTOP-123 (or some number at the end). This keeps happening as I block more and more devices.
• Device is connected through Ethernet cable.

Is there a way to block a device by "Hostname starts with" = "NEW-DESKTOP" for instance? Cause all devices have the NEW-DESKTOP-NNN format.

When I was managing the device through the Gateway interface (192.168.0.1), I never had this issue and was able to block that device from the network and even schedule the time it's blocked, but now that Cox moved managing devices to wifi.cox.com, I can't seem to be able to block it anymore.