server lesed an ip address after it had been given a hostnam

i gave hostnames for my lan computers starting from 192.168.0.10 and for my surprize i found that 192.168.0.10 had been leased for a computer.
i cleared my leases after creating hostnames for every internet user(computer).
there are exactly the same number of leases as there are host names.
That computer has no user account in my server.

basicly i want to ban this computer from accessing the net from my lan.

is there a way i can ban this computer from accessing the net?

thanks for any help
sander