For ipv6 only customers, just enter zero in number of IP's in master config, enter 1 (or whatever you want to give) for number of IPv6 ip's, and then, delete all ip pools except the ipv6 you want to use.
You need to completely disable IPv4 in the plan configuration (not just set to 0), then only add the IPv6 subnet. If you tell me which control panel you use (eg. SolusVM, Proxmox…), I can give you more details.