Unless I'm all wet:
1 - Disconnect your firewall from the LAN
2 - Set the lease time to 1 minute (you might have to stop and restart the DHCP service)
3 - wait a little over more than 1 minute
4 - The firewall clears its own lease table
5 - Set the DHCP back to the normal lease time
6 - Reconnect the LAN
Unless I'm all wet.
CS...