There is no direct way to disable the 2.4 GHz radio (or 5GHz radio) from AP groups. However, you can create a "WLAN Group" and uncheck all the WLANs in that group. Next, go to the AP group and select the WLAN Group under Radio Settings :: Radio B/G/N (2.4 GHz). This will effectively disable all the WLANs in 2.4 GHz radio for all those APs that are part of that AP group.
I hope this helps.