Or you could just delete the lease and force a renew.
If you don't want to wait for the 1/2 lease time, you could use telnet or psexec to remotely force the client to renew by running ipconfig /renew remotely.
http://technet.microsoft.com/en-us/sysinternals/bb897553.aspxSay a machine has 192.168.10.195, delete the lease from DHCP, and run the following command:
psexec \\192.168.10.195 ipconfig /renew
The client will be forced to get a new address because of the exclusion.