This supposed to be available in the Network → Wireless section.
I have no such option, which I am guessing is because there is no WiFi card in the machine. The WiFi is through an access point connected to one of the ethernet ports.
Is there a way to configure it so the access point has the same option.
Switch to router mode, assign an IP address to the LAN and disable the DHCP server. Connect the device to the main router using some of the lan ports and do not use the wan port.
This will work if you want to restrict only the Internet access. If you want to restrict also the access to the wired lan, you will need to use a different IP subnet for the wireless clients.