As of now there is no GUI way to do that. You will have to go into IAP CLI and perform a disconnect.
Monitoring > Access Points > List > Look for the Master IAP highlighted in bold.
Click on the Master IAP, on the right top corner is Console button.
Console will prompt you for username (probably admin) and password (the group password).
THen you can execute any of the below:
disconnect-user ?
<addr> addr
all
mac
network
PS: The UI change to do this is under works.