Problem with Dhcp Server

hi,

i am running dhcp server to assign automatic ip from pool , i m having problem with some users from LAN , that, there mac address getting all ip address which r left on DHCP server, and making problem on whole LAN , i think this is some kind of virus attacking on dhcp, i cannot able to block those mac address manually because some one doing it for purpose by changing MAC address frequently.

please help

All IP Address Means all the remaining IP address..???

yes, all remaining ips in that subnet