Terminate a Process
Sometimes a process might need to be stopped. Maybe the force quit doesn't work or maybe the process is a background process which isn't shown in the list. In these cases the command line can be used to terminate the process. CAUTION: don't terminate processes unless you really know what you are doing!
First you need to find out the process ID. To do this use the ps or top command. See the related commands section for information about these. Use ps -axc and find the process ID of the required process (its the number in the first column). Now use the kill command to terminate the process, type kill xxx where xxx is the number of the process to kill.
Often the process you want to kill doesn't belong to you so you will get an error message "Operation not permitted". To kill the process anyway use the sudo utility. Type sudo kill xxx, and enter an administrator's password when asked. Check the process no longer exists by using ps again. Some system processes automatically restart after being terminated but any problems should have been cleared by the restart. Occasionally a process won't respond to a standard kill, so use sudo kill -9 xxx instead.
More information: Getting Started.