If you want to block an IPv6 IP then follow the below steps, 

First of all, click on the Block IPs from the side menu in the app,

Start From Here:-

A) Click on the IPv6 IPs button from the menu

B) Now click on the ADD NEW IP button

After that a form will be opened, Then do the following steps:-

1) Enter IPv6 IP=> Add the IPv6 IP 

2) Now click on the SAVE TO BLOCKED LIST button to add the IP to the Blocklist

If you want to delete/edit The IPv6 IP:-

1) Click on the Trash/Edit icon to delete/edit the IP from the Blocked list