Go to Network-wide > Monitor > Clients.
Check the boxes of the clients that you want to block.
Click on the Policy drop-down above the client list, and select Blocked.
This will immediately block the selected clients from accessing the internet. Please note that this configuration is completed on a client-by-client basis.
If you want to block all devices by default and only allow specific devices, you can create a Deny Any rule in the Layer 3 firewall. Then, you can add the MAC addresses of the devices you want to allow to your device.
Blocking and Allowing Clients - Cisco Meraki
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
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