What do I need to do to disable IPv6?

I'll never use IPv6 and want to disable it completely. I don't like having rpcbind or ssh running on both v4 and v6.
I've tried disabling using sysctl but getting errors in the logs from pve-firewall:

iptables_restore_cmdlist: Try `ip6tables-restore -h' or 'ip6tables-restore --help' for more information.

Is there a guide anywhere on how to go about removing v6 support?