If you're in the preliminary phase of building a SSID, you can do this via the GUI interface under EXPRESS SECURITY SETUP by deselecting broadcast SSID in Beacon. If you have the SSID in production, then go to your CLI...priveleged exec mode, and enter the following:
ap#conf t
ap(config)#dot11 ssid yourssidname (duh)
ap(config-ssid)#no guest-mode
this makes the beacon remove the ssid from its broadcast and requires a client to know the ssid in order to configure their wlan card to participate.
