Could you specify what you mean with "Directadmin's iptables"?
Because DA does not have any iptables. Iptables is the firewall which is part of the operating system.
After a reboot, it's restarted which clears all lists.
If you want a firewall which remembers blocked ip's after a reboot, best thing you can do is install a shell for it like Configserver Firewall, better known as CSF/LFD. You can find several items about it on the forums here. There is also a DA plugin for it.