Chapter 10 Install and System Administration for FortiOS 5.0 : Advanced concepts : DHCP servers and relays : Breaking a address lease
  
Breaking a address lease
Should you need to end an IP address lease, you can break the lease using the CLI. This is useful if you have limited addresses, longer lease times where leases are no longer necessary. For example, with corporate visitors.
To break a lease enter the CLI command:
execute dhcp lease-clear <ip_address>
See Also
DHCP Server configuration
Service
DHCP options
DHCP servers and relays