Or you could go down the individual AP route (lots of work if you have large number) and untick the "enable WLAN service for this radio"
I would assume this turns off the 5GHz. Assumptions can be wrong!
From help manual:
WLAN Service: Uncheck this check box to disable WLAN service entirely for this radio. (This option can be useful if you want dual-band 802.11n APs to provide service only on the 5 GHz radio, in order to reduce interference on the 2.4 GHz band, for example.) You can also disable service for a particular WLAN at specific times of day or days of the week, by setting the Service Schedule. For more information, see
Advanced Options for creating a WLAN.
screenshots