Deny DHCP IP to a certain MAC?

[Phil]

Hi,

I am running DHCP (also with DDNS) on Fedora Core 2.

I am getting some instances of blank hostnames getting an IP
address... I want to prevent those people from renewing their IP.

So I actually have a 2 questions;

1- Can I prevent blank host names from getting a DHCP address? (I know
I can deny unknown hosts, but that would require me to enter in all my
known hosts manually, somethign I do not want to do, since there is
over 250 of them)
2- Since I know their MAC addresses is there a way to deny them a IP from DHCP?

Thanks!