xFi Gateway deny unknown MAC addresses

I have the XFi Gateway and trying to use downtime rules to shut off the internet at bed times for my children. My teenage son has already discovered that he can change the MAC address of the Network adapter in the PC to bypass the downtime rules. The issue is unknown devices are allowed with no restrictions. Is there a way to setup downtime’s for unknown devices or restrictions for unknown devices? I would prefer unknown devices have no access at all until I profile them. Maybe I’m missing a setting option or this is a serious flaw in the profiling capabilities in the xFi gateway. Thanks