You are correct. You need to configure this over CLI.
Here's the CLI guide for FW 9.6
https://support.ruckuswireless.com/do...
There on page 247 you will find the answer and, if I'm not mistaken it goes like this
> enable
# config
# wlan your_wlan_name
# ofdm-only
# exit
To check if 11b has been disabled
# wlan your_wlan_name
# show
look for "OFDM-Only State" to be ENABELED.
You enable it per SSID (WLAN). So you have to repeat for every SSID you create.