If you are being attacked or abused by a particular host, just enter the following command to deny all access to the host:

/sbin/route add -host  reject

Replace with the IP address of the host you want to block. The address will stay blocked until you bounce the interface. Alternatively, you could allow access by:

For a more permanent rejection, of course, use your firewall. If you really want this to persist, add the command to rc.local or whatever your favorite is.