How to block outside initiated traffic in NPTv6? NPTv6 does prefix translation between one IPv6 address to another. Is there a way to prevent outside initiated traffic to inside (preferably in A10 AX devices)?

In order to implement an "outgoing connections only" policy the firewall needs to keep track of the connections initiated internally so it can allow responses back in without opening up the floodgates. There is no fundamental differance here between IPv4 and IPv6.

Whether your particular hardware supports that for IPv6 I cannot answer.