I blocked a MAC address of a client on my DHCP Server running on Ubuntu 12.10 using
iptables -A INPUT -m mac --mac-source -j DROP
But I am still getting DHCP requests from the client and the client is getting the IP address from my DHCP server too. I think the command does not work for broadcast messages. Is there any other technique to do it? I do not have access to the DHCP Server code.