Hi Paddy,
Unfortunately there is no way to kick a client off the radius server, and since the On-Site ZD and Ruckus SCG check the radius server for the client's mac address there is it will always return a success after the first attempt.
One way around it is to create multiple portal, generating different Radius server and testing the different log in option by using a different portal/Hotspot service. It's not a straight forward as being able to delete/removing a client mac, but it will allow you to test different method of authenticating a client. Just change the HotSpot services on the single test wlan.
Sid