ip leases with no host name ms server 2003

hi, the address leases in my dhcp show a few ip's that have no description or name, just the ip address, i did a reverse lookup to find out the name of the computer but it just doesn't exists. i got the MAC address but not the host name
the only reazon why i'm worried is because that address is consuming 22% of my bandwidth, i killed it from dhcp but i'm afraid it will come up again

any thoughts?