One of the reasons why I decided to move back to IPV4 is that Digitalocean blocks all SMTP outgoing connections on port 25 over IPV6. I do regret a lot about switching to IPV6 after I went through a long tutorial here on DO because it consumed me a lot of time just to figure out later that my WordPress site and any other application completely stopped sending emails.
That's a shame DO haven't solved this issue already. DO should make it clear it'd block all and every connections, protocols and ports need by sendmail, postfix etc.
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.