Always try to kill a process with a simple kill command.
kill process_identifier
This is the cleanest way to kill a process and has the same effect as cancelling a process.
If this does not work use the -1 (one) signal
kill -1 process_identifier
This tells the process to hangup just as though you were logging out. The system will attempt to kill any child processes.
As a last resort use the -9 signal:
kill -9 process_identifier
This will stop the process dead in its tracks but it may leave any child processes still running.