How to block a MAC address in the DHCP?

hi,
I have a DHCP server and some unknown users are connected to it. I want to block them through MAC address is there any way? or is there any way to block a particular MAC address on my network?
I have a DHCP server and some unknown users are connected to it. I want to block them through MAC address is there any way? or is there any way to block a particular MAC address on my network?
Comments
Otherwise yup as Dynamik said associate to a blocked IP
DHCP Server Callout DLL for MAC Address based filtering - Microsoft Windows DHCP Team Blog - Site Home - TechNet Blogs and if i remember correctly it works for 32bit only.
the site says it works with 2008 R2 which is 64 bit.
certs: MCITP (7 and Vista),MCTS,MCSE,MCSA,MCDST,CCAI,A+,Network+,Server+
Is it safe for my DHCP server?
I found this article that has a video on how to do just that. It works on a WIndows 2003 Server.
Block Mac Address
DHCP Block List - Callout DLL - Tech and Web Guy