Cisco Wireless :: 1550AP - Define Area Without WLAN Coverage?

i have an Outdoor Area covered by Cisco Wireless LAN Solution (WCS,WLC,1550AP root and Mesh lightweight). I need to disable power transmission for some of these APs in the 2..4GHz.I tried two possible way, but both are not working:
 
1) Set the power of 802.11g/g/n to 0. There is no such option (power can be set manually from 1 to 5 and, as per the documentation, each level is a reduction of 3dB.  Therefore, even if i put to the minimum, i cannot guarantee that there is no utilization of the 2.4GHz spectrum).
 
2) Define an AP group containing these APs and don't add any SSID. Is not possible because at least one SSID must be added on each AP group.
 
Working solution/setting in AP/WLC/WCS to avoid transmission in 2.4GHZ? Otherwise the only solution is to power them off?