From the CLI: set service-profile XXXXXXXX beacon disable
save config
Substitute the service profile name where the XXXXX is.
If you use WMS (or Ring Master, as appropriate), you can turn off beaconing under the service profile properties.
Keep in mind that if you have a SSID not broadcast that some clients such as Windows Vista PCs, might not attach properly. If you're using the SSID for wireless voice, you could have problems as well. You don't gain much by not broadcasting the SSID.