If a process is shown with process state "D" in tools like ps or top, then its a zombie process. Example:
ps axl | grep D F UID PID PPID PRI NI VSZ RSS WCHAN STAT TTY TIME COMMAND 4 104 2579 9109 20 0 5024 2116 refrig D ? 0:00 cleanup -z -t unix -u -c
Zombie processes can not be killed with the normal commands like "kill -9 PID", you will have to reboot the system to kill them.