There are many reasons to pull Ipv6 support from Proxmox (and Debian)

So here's how to do it. 

There are eseentially two options

GRUB

The first involves adding 

ipv6.disable=1 

to the GRUB_CMDLINE_LINUX and GRUB_CMDLINE_LINUX_DEFAULT lines in 

/etc/default/grub

and then running 

update-grub

before rebooting - Effective and brutal. 

SYSCTL

The safer way, is to use the sysctl commands to turn it off in the network stack, and that is done by creating a file /etc/sysctl.d/60-disableipv6.conf

Add the following lines to that file: 

net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1

Then activate this configuration using

sysctl --system 

Remember to sudo if you're not a superuser.