Just create an empty WLAN Group (a WLAN Group without any WLAN object) and set this WLAN group as override on the 2,4 GHz section in the AP Group configuration. This is how we disabled 2.4 GHz for certain AP groups.
It's a little cumbersome to restrict SSIDs to certain
bands. You need to create a "WLAN Group (WG)" to advertise
a subset or specific set of SSIDs. For example:
Create a WG called "Public-Guest" in your
Zone (under Wireless LANs > Select Zone > "+" to create a
Move the JUHSD-Public SSID into this WG
Under Access Points, select a Zone and Access Point
Group (AG) to associate with this WG. This may be the default AG
unless you've grouped APs into logical groups.
Edit the AG using the pencil icon above. Under
"Radio (2.4 GHz)" > WLAN Group, move the slider to
"ON" to override the group and select the WG you created
earlier. This will only advertise SSIDs in this WG on the 2.4GHz