sudo sed -i '/^extraargs=/$/ ipv6.disable=1/' /boot/armbianEnv.txt
Respectively add ipv6.disable=1 to the end of the "extraargs" line, separated with a space from other possible values.
However, this disables IPv6 support completely kernel-wise, which some software has problems with. E.g. the default Apache2 config on Debian comes with a vhost which dynamically binds to IPv4 and/or IPv6 addresses based on whether it is active on the interface or not. This syntax requires the kernel to at least understand IPv6, otherwise the webserver fails to start. So it is generally safer to only disable IPv6 on all interfaces, but keeping the kernel module active:
echo -e 'net.ipv6.conf.all.disable_ipv6=1\nnet.ipv6.conf.default.disable_ipv6=1' | tee /etc/sysctl.d/disable_ipv6.conf
sudo sysctl -p /etc/sysctl.d/disable_ipv6.conf
EDIT: Ah, now I see you basically tried this already. As this is universally functional on all Linux distros, it must actually work. Not sure how exactly to tried it, which exact command or config file? E.g. using the sysctl command itself is not boot-persistent, hence the config file.
