Since you haven't noted what OS you're using, so I'm guessing it's Ubuntu/Debian.
Go and edit this file /etc/network/interfaces.
And remove the IPv6 section that looks similar to this and then reboot the server:
iface eth0 inet6 static
address primary_ipv6_address
netmask 64
gateway ipv6_gateway
autoconf 0
dns-nameservers 2001:4860:4860::8844 2001:4860:4860::8888 209.244.0.3
It's not possible to remove the "IPv6" nor "Private Networking" from the droplet via the control panel after it's been activated.
But by removing the configuration, we actually disable the functionality completely.
You misunderstand my question. OS is irrelevant. Each droplet has an option in the control panel to enable IPv6. Once you enable it, there is no option to disable it.
You didn't read the last part of my comment:
It's not possible to remove the "IPv6" nor "Private Networking" from the droplet via the control panel after it's been activated.