Thanks. That works - seems that IPv6 is very tightly integrated, if that is the only way to completely disable it.
With regards to your question: My whole LAN is currently using static IPv4 addresses and comprises only a few computers. Thus there is no benefit from having IPv6 up and running - on the contrary, I can stick with the current firewall setup as I do not need to set up rules to tame IPv6. And I am a fan of the principle of minimizing the attack surface by disabling / uninstalling any piece of software / service that I do not need.