Hi,
Try to do this:
Go to Configuration> Wireless> AP Configuration. Edit your ap group. Expand Wireless Lan, expand Virtual AP and click on the virtual AP with your ssid. In the right Pane you should enable"Drop Broadcast and Multicast" and "Convert Broadcast ARP requests to unicast". Click on apply in the lower right corner.
When I enable these settings this message appears "Warning broadcast-filter arp should be enabled with this option. Otherwise ARP requests will be dropped".
it's more then recomended that to always turn on Drop Broadcast and Multicast, Convert Broadcast ARP requests to unicast and broadcast-filter ARP to have a more realible network.
turn this on in all vaps + mark -> broadcast filter ip on the stateful firewall.
it should reduce the broadcast the clients are using over you Wi-Fi deployment.
Let us know if it solved your issue.
me.