You can't close sockets without shutting down the process that owns those sockets. Sockets are owned by the process that opened them. So to find out the process ID (PID) for Unix/Linux. Use netstat like so:
netstat -a -n -p -l
That will print something like:
Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name
tcp 0 0 127.0.0.1:25 0.0.0.0:* LISTEN 1879/sendmail: acce
tcp 0 0 0.0.0.0:21 0.0.0.0:* LISTEN 1860/xinetd
Where -a prints all sockets, -n shows the port number, -p shows the PID, -l shows only what's listening (this is optional depending on what you're after).
The real info you want is PID. Now we can shutdown that process by doing:
If you are shutting down a service it's better to use:
service sendmail stop
Kill literally kills just that process and any children it owns. Using the service command runs the shutdown script registered in the init.d directory. If you use kill on a service it might not properly start back up because you didn't shut it down properly. It just depends on the service.
Unfortunately, Mac is different from Linux/Unix in this respect. You can't use netstat. Read this tutorial if you're interested in Mac:
And if you're on Windows use TaskManager to kill processes, and services UI to shutdown services. You can use netstat on Windows just like Linux/Unix to identify the PID.