Click the VNS tab at the top of the controller GUI, and then Roles on the left side panel.
Select the role you wish to block IPV6 on, and then select policy rules. A popup will appear:
On the third pane, under layer 2 select version 6 (IPV6). In the action pane, use the pull-down menu to select deny, then click ok, the popup will disappear. On the next screen hit save on the bottom right to save your changes.