How to stop adding a device through the Ethernet cable?

Hi,

I'm trying to limit the screen time for my children as I assinged their devices from Xfinity App. on my phone.

However, recently my son got the Ethernet Cable to connect his computer and somehow he was able to unassign his computer and added his computer again as a new device, so he can ditch my assigned time.  Now, he's playing game all night long withoust noticing me.  Can you just pause the Wifi tower itself during night time?  If I can do so, please explain how to do this?  Also, he was mentioning that he is able to hide his IP address (or his computer name), so he would be able to use his computer without noticing me, also.