The only way to clear dhcp leases for ipv4 or ipv6 is to edit the lease table. I really wish they would fix this. It should be possible to leases to be cleared and to force leases to be renegotiated.
I had some visitors over last night and there were a few leases left over, three dhcp and one dbcp6. Disabling and enabling dhcp6 cleared the lease. Disabling and enabling dhcp cleared one of three leases, but didn't clear the other two.
The only way I could remove all of the dhcp leases was to remove them from /var/chroot-dhcps/var/state/dhcp.leases.