I know MAC address filtering is under consideration, but I have a similar recommendation that may make more sense for less technical users: The ability to lock down the network once the user has all of their devices connected. Once this lock is turned on (via a toggle in the app), no devices will be added to the network that are not currently connected or have not connected previously. To allow devices to be added again, the user would simply need to toggle the lock off.
This would be separate from guest devices of course which could be added on the guest network, but for a household, devices do not change that often (usually only after a purchase, replacement, etc.). It adds some additional security without requiring users to worry about typing in MAC addresses, etc.