Navigate to Network-wide > Monitor > Clients, then check the boxes of the clients that you want to allow list or block. Click on the Policy drop down above the client list, and select blocked or allow listed. To apply the allow list or block on a per-SSID basis or only on the MX security appliance, select Group policy.
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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