In latest codes, you can do it on the Virtual AP
Aruba7240) # show wlan virtual-ap test
Virtual AP profile "test"
-------------------------
Parameter Value
--------- -----
<------ truncated ------>
Blacklist Time 3600 sec
Deny inter user traffic Disabled
Deny time range N/A
In old codes and also to do in global try in firewall.
(Aruba7240) #show firewall | include eny
Deny all IP fragments Disabled
Deny inter user bridging Disabled
Deny inter user traffic Disabled
Deny source routing Disabled