Oh, it's really simple. Wireless access points work by bridging the
wireless port to the wired switch ports and router port. Everything
happens at the MAC address level and does not involve IP addresses,
NETBIOS over TCP/IP (also known as MS Networking). Just MAC
addresses.
The wireless bridge builds a bridging table consisting of a table of
"heard" (or sniffed) MAC addresses that appear on various ports.
There are really just 3 available ports[1]. Wireless, ethernet
switch, and router port. If the destination MAC address of a port is
shows up in the MAC address table as sitting on a specific port, only
that port gets the traffic. Broadcasts, which have no destination MAC
address are sent to all ports.
Well, it's simple enough to build a logical rule (or filter) for these
MAC addresses and ports that says:
"If the packet originates on the wireless port, it can only send
and receive packets that are destined or originate from the router
port or ethernet switch port."
Not a very complex rule, but one which totally prevents wireless
client to client traffic. Not even broadcasts will go from wireless
client to client.