I have discovered how to accomplish this with the SZ-100. I have 3 SSIDs, Lets just call them WiFi 2.4GHz only, WiFi 5GHz only and WiFi Auto. Credit to me figuring out the first part goes to Christopher Scott at:
http://blog.chrisjscott.net/2015/09/08/forcing-clients-to-use-5ghz-on-a-ruckus-wireless-network/.
I created the 3 WLANs with their SSIDs being for example: "WiFi 2_4GHz only", "WiFi 5GHz only" and "WiFi Auto". (if you are trying to modify an existing WLAN then just clone it and just change the SSID of the/each new WLAN)
Then added two WLAN Groups – one called “2.4GHz” and another called “5GHz”
Then specify which WLANs should be a member of which group. For this example set "WiFi 2_4GHz only" as a member of "2.4GHz", "WiFi 5GHz only" as the member of "5GHz" and "WiFi Auto" as a member of both "2.4GHz" and "5GHz".
Finally, go to Access Points then in the tree, go to System -> Default Zone -> default (or whichever access group your access points are in) and edit that Access Group and set the WLAN Group override setting to ON then use the WLAN Group drop-down menus to specify each of your WLAN Groups to associate with each radio band.
Because the "WiFi Auto" SSID is a member of both groups, anyone connecting to that SSID can be automatically transitioned between the radio bands while the other 2 SSIDs are members of only their respective group so they will be restricted to only the band of which the SSID is a member.