LMNOP wrote

My son has worked he can change the MAC address to bypass parental controls. This means my deco is essentially useless - the whole reason I spent all that money on this system was to control his screen time. There's no whitelist, and there is no manual

approval of new devices joining the network. I have to manually block each new MAC address when he connects. My only option is to change the password, reset the connections for 20+ devices and smart home devices, and hope that his siblings don't tell him the new password. This is a terrible solution, and I can see that there have been people raising this as a problem in this forum for years now.

  @LMNOP 

Not an ideal solution I will grant you, but have you thought about restricting access of his logon by making sure he doesn't have the access permission required to change the MAC address.

I have the same problem with my grandson except in my case the computer he is using is not under my control so unfortunately I cannot implement my own suggestion :-( 

The best solution that I can come up with is to shut down my Wi-Fi system when I go to bed.

Not very elegant, but it does work