Pupil access to Unifi Guest network

[maxrebo]

Is there any way on Unifi to stop pupils taking a ip Address before they attempt to join our Guest SSID?

I've setup a Guest network using a portal and voucher based authentication, only problem is the students see an open Wifi and try to join it, it then gives them a IP and when the get to the redirected portal they then give up as they don`t have a Voucher.

 

I know on Ruckus you used to be able to stop them taking an IP address until they authenticated (or at least releasing it once they couldn't authenticate) Can you do this on Unifi?