Killing zombie processes in Linux or Unix cannot be done, as they are already dead. 🙂 Often the only solution is to restart your machine. However, it is sometimes possible to kill or restart the parent process which will then often clear out zombie child processes.
To show parent/child process hierarchy, use the following command:
$ ps auxwwf
You will see output similar to the following. If any child process show up as zombies, you can either kill or restart the parent process. ($ sudo kill -9 PID) Otherwise, your only option may be a reboot.