Any idea how to release a DHCP client?

does anyone know how to release a DHCP lease from the server (panther server)? Even after the client has shut down, that client is still assigned a specific IP address, and I want to release the lease from the server. I've tried stopping the server and also rebooting the server, but the lease is still there! I've even tried using NetInfo to tie the MAC address of the client to a different IP address. Doesn't work. Please help. I'm stuck.