There is a feature available on the AP (on the "Association Table Advanced" Page) to prevent wireless clients from talking to each other **on the same subnet** (not on the same "wireless network" as it says below).
It is called PSPF and is documented here;
http://www.cisco.com/univercd/cc/td/doc/product/wireless/airo_350/accsspts/ap350scg/ap350ch3.htm#1134993
Here is what the documentation says;
Block ALL Inter-Client Communications (PSPF)
Publicly Secure Packet Forwarding (PSPF) prevents client devices associated to an access point from inadvertently sharing files with other client devices on the wireless network. It provides Internet access to client devices without providing other capabilities of a LAN. With PSPF enabled, client devices cannot communicate with other client devices on the wireless network. This feature is useful for public wireless networks like those installed in airports or on college campuses.
Note The PSPF feature is available in firmware versions 11.08 and later, which are available on Cisco.com. You can download Cisco Aironet firmware releases at http://www.cisco.com/public/sw-center/sw-wireless.shtml.