It appears the n-only, ac-only and now n/ac/ax-only mode is configurable on the ZoneDirector and Unleashed platforms.
SmartZone has an OFDM-only option, which excludes 802.11b clients.
Further information here: https://support.ruckuswireless.com/articles/000004392
Hope that helps,
You can disable 802.11b by enabling OFDM but not others though you can try setting a mode type and see if that helps you with your requirement. You can use below command on AP and if that serves the purpose create AP CLI script and push to all connected AP's.
rkscli: set mode
Commands starting with 'set mode' :
set mode : set mode <wifi name> <radio mode>
Valid modes :11ahe , 11ac ,11na ,11ng, 11ghe