The short form answer is limit the DHCP pool @ GL GUI -> LAN -> DHCP Server for random devices & use 'Address Reservation' — aka static leases — for any dedicated devices (eg: a NAS, IP cameras, computers, etc.). Combined that with the Clients options for the MAC control.
For another setup using advanced set up for further fine tuned control: quarantine 'em even further. Put'em on a Guest/dedicated VLAN. Make sure they know they have to expose their real MAC to use it, assign static IP leases of 'infinite' (LuCI -> Network -> DHCP & DNS -> Static Leases) & limit the number of IPs that DHCP pool that VLAN hands out. No real MAC? No Wi-Fi to WAN!
This HOW-TO will also work on your Flint v2. Adapt as required. Enjoy: