Wikipedia
Tcpkill is a network utility program that can be used to kill connections to or from a particular host, network, port, or combination of all. These programs take standard Berkeley Packet Filter (BPF) filters. This can be used for both port mirroring and arp spoofing
For example, to prevent any connections to the host www.google.com use this command:
/usr/sbin/tcpkill −9 host www.google.com
The computer that is attempting to go to that site will be blocked from that site only, but can surf any other site. It is a good idea to either redirect the output into nothingness ( > 2>/dev/null 1>/dev/null) or into a file for later analysis (> file.tcpkill ). By default, it will redirect output to the console.
More hosts can be specified with the command:
/usr/sbin/tcpkill −9 host www.google.com and host www.yahoo.com
To block well−known ports e.g., napster (port 8888 and port 6699) or gnutella (port 6346), the command:
/usr/sbin/tcpkill −9 port 8888 and port 6699
or
/usr/sbin/tcpkill −9 port 6346 --> will do the trick.